title: NPE in Drag and Drop
body: <p>I'm tested this code in Windows JVM 7_25 it works well.</p>
  
  <pre><code>import java.util.Random;  
  import javafx.application.Application;  
  import javafx.collections.ObservableList;  
  import javafx.event.EventHandler;  
  import javafx.scene.Node;  
  import javafx.scene.Scene;  
  import javafx.scene.input.ClipboardContent;  
  import javafx.scene.input.DragEvent;  
  import javafx.scene.input.Dragboard;  
  import javafx.scene.input.MouseEvent;  
  import javafx.scene.input.TransferMode;  
  import javafx.scene.layout.BorderPane;  
  import javafx.scene.layout.FlowPane;  
  import javafx.scene.layout.Pane;  
  import javafx.stage.Stage;  
  public class DraggableFlowPane extends Application {  
      @Override  
      public void start(Stage primaryStage) {  
          final FlowPane root = new FlowPane();  
          final Random rng = new Random();  
          final int NUM_NODES = 120;  
          for (int i = 0; i &lt; NUM_NODES; i++) {  
              int red = rng.nextInt(256);  
              int green = rng.nextInt(256);  
              int blue = rng.nextInt(256);  
              Node node = createNode();  
              node.setStyle(String.format("-fx-background-color: rgb(%d, %d, %d);", red, green, blue));  
              root.getChildren().add(node);  
          }  
          primaryStage.setScene(new Scene(root, 600, 500));  
          primaryStage.show();  
      }  
      private Node createNode() {  
          final BorderPane bp = new BorderPane();  
          bp.setOnDragDetected(new EventHandler&lt;MouseEvent&gt;() {  
              @Override  
              public void handle(MouseEvent event) {  
                  Dragboard db = bp.startDragAndDrop(TransferMode.MOVE);  
                  ClipboardContent clipboard = new ClipboardContent();  
                  final int nodeIndex = bp.getParent().getChildrenUnmodifiable()  
                          .indexOf(bp);  
                  clipboard.putString(Integer.toString(nodeIndex));  
                  db.setContent(clipboard);  
                  event.consume();  
              }  
          });  
          bp.setOnDragOver(new EventHandler&lt;DragEvent&gt;() {  
              @Override  
              public void handle(DragEvent event) {  
                  boolean accept = true;  
                  final Dragboard dragboard = event.getDragboard();  
                  if (dragboard.hasString()) {  
                      int incomingIndex = Integer.parseInt(dragboard.getString());  
                      int myIndex = bp.getParent().getChildrenUnmodifiable()  
                              .indexOf(bp);  
                      if (incomingIndex == myIndex) {  
                          accept = false;  
                      }  
                  } else {  
                      accept = false;  
                  }  
                  if (accept) {  
                      event.acceptTransferModes(TransferMode.MOVE);  
                  }  
              }  
          });  
          bp.setOnDragDropped(new EventHandler&lt;DragEvent&gt;() {  
              @Override  
              public void handle(DragEvent event) {  
                  boolean success = false;  
                  final Dragboard dragboard = event.getDragboard();  
                  if (dragboard.hasString()) {  
                      int incomingIndex = Integer.parseInt(dragboard.getString());  
                      final Pane parent = (Pane) bp.getParent();  
                      final ObservableList&lt;Node&gt; children = parent.getChildren();  
                      int myIndex = children.indexOf(bp);  
                      final int laterIndex = Math.max(incomingIndex, myIndex);  
                      Node removedLater = children.remove(laterIndex);  
                      final int earlierIndex = Math.min(incomingIndex, myIndex);  
                      Node removedEarlier = children.remove(earlierIndex);  
                      children.add(earlierIndex, removedLater);  
                      children.add(laterIndex, removedEarlier);  
                      success = true;  
                  }  
                  event.setDropCompleted(success);  
              }  
          });  
          bp.setMinSize(50, 50);  
          return bp;  
      }  
      public static void main(String[] args) {  
          launch(args);  
      }  
  }
  </code></pre>
  
  <p>When I tested the same code on Linux with JVM 7_25 I get this error stack:</p>
  
  <pre><code>    Executing com.javafx.main.Main from /home/rcbandit/Desktop/test/DX-57DC/dist/run652386450/DX-57DC.jar using platform /opt/jdk1.7.0_25/bin/java  
      Exception in thread "JavaFX Application Thread" java.lang.NumberFormatException: null  
          at java.lang.Integer.parseInt(Integer.java:454)  
          at java.lang.Integer.parseInt(Integer.java:527)  
          at com.dx57dc.main.DX57DC$2.handle(DX57DC.java:58)  
          at com.dx57dc.main.DX57DC$2.handle(DX57DC.java:52)  
          at com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:69)  
          at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:217)  
          at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:170)  
          at com.sun.javafx.event.CompositeEventDispatcher.dispatchBubblingEvent(CompositeEventDispatcher.java:38)  
          at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:37)  
          at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:92)  
          at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:35)  
          at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:92)  
          at com.sun.javafx.event.EventUtil.fireEventImpl(EventUtil.java:53)  
          at com.sun.javafx.event.EventUtil.fireEvent(EventUtil.java:33)  
          at javafx.event.Event.fireEvent(Event.java:171)  
          at javafx.scene.Scene$DnDGesture.fireEvent(Scene.java:2631)  
          at javafx.scene.Scene$DnDGesture.processTargetEnterOver(Scene.java:2780)  
          at javafx.scene.Scene$DnDGesture.access$6100(Scene.java:2607)  
          at javafx.scene.Scene$DropTargetListener.dragEnter(Scene.java:2523)  
          at com.sun.javafx.tk.quantum.GlassSceneDnDEventHandler$1.run(GlassSceneDnDEventHandler.java:63)  
          at com.sun.javafx.tk.quantum.GlassSceneDnDEventHandler$1.run(GlassSceneDnDEventHandler.java:54)  
          at java.security.AccessController.doPrivileged(Native Method)  
          at com.sun.javafx.tk.quantum.GlassSceneDnDEventHandler.handleDragEnter(GlassSceneDnDEventHandler.java:54)  
          at com.sun.javafx.tk.quantum.GlassViewEventHandler.handleDragEnter(GlassViewEventHandler.java:395)  
          at com.sun.glass.ui.View.handleDragEnter(View.java:692)  
          at com.sun.glass.ui.View.notifyDragEnter(View.java:1005)  
          at com.sun.glass.ui.gtk.GtkDnDClipboard.pushToSystemImpl(Native Method)  
          at com.sun.glass.ui.gtk.GtkDnDClipboard.pushToSystem(GtkDnDClipboard.java:24)  
          at com.sun.glass.ui.SystemClipboard.flush(SystemClipboard.java:28)  
          at com.sun.glass.ui.ClipboardAssistance.flush(ClipboardAssistance.java:34)  
          at com.sun.javafx.tk.quantum.QuantumClipboard.flush(QuantumClipboard.java:197)  
          at com.sun.javafx.tk.quantum.QuantumToolkit.startDrag(QuantumToolkit.java:1195)  
          at javafx.scene.Scene$DnDGesture.dragDetectedProcessed(Scene.java:2652)  
          at javafx.scene.Scene$DnDGesture.process(Scene.java:2713)  
          at javafx.scene.Scene$DnDGesture.access$8700(Scene.java:2607)  
          at javafx.scene.Scene$MouseHandler.process(Scene.java:3344)  
          at javafx.scene.Scene$MouseHandler.process(Scene.java:3168)  
          at javafx.scene.Scene$MouseHandler.access$1900(Scene.java:3123)  
          at javafx.scene.Scene.impl_processMouseEvent(Scene.java:1563)  
          at javafx.scene.Scene$ScenePeerListener.mouseEvent(Scene.java:2265)  
          at com.sun.javafx.tk.quantum.GlassViewEventHandler$MouseEventNotification.run(GlassViewEventHandler.java:250)  
          at com.sun.javafx.tk.quantum.GlassViewEventHandler$MouseEventNotification.run(GlassViewEventHandler.java:173)  
          at java.security.AccessController.doPrivileged(Native Method)  
          at com.sun.javafx.tk.quantum.GlassViewEventHandler.handleMouseEvent(GlassViewEventHandler.java:292)  
          at com.sun.glass.ui.View.handleMouseEvent(View.java:528)  
          at com.sun.glass.ui.View.notifyMouse(View.java:922)  
          at com.sun.glass.ui.gtk.GtkApplication._runLoop(Native Method)  
          at com.sun.glass.ui.gtk.GtkApplication$3$1.run(GtkApplication.java:82)  
          at java.lang.Thread.run(Thread.java:724)  
      Exception in thread "JavaFX Application Thread" java.lang.NumberFormatException: null  
          at java.lang.Integer.parseInt(Integer.java:454)  
          at java.lang.Integer.parseInt(Integer.java:527)  
          at com.dx57dc.main.DX57DC$2.handle(DX57DC.java:58)  
          at com.dx57dc.main.DX57DC$2.handle(DX57DC.java:52)  
          at com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:69)  
          at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:217)  
          at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:170)  
          at com.sun.javafx.event.CompositeEventDispatcher.dispatchBubblingEvent(CompositeEventDispatcher.java:38)  
          at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:37)  
          at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:92)  
          at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:35)  
          at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:92)  
          at com.sun.javafx.event.EventUtil.fireEventImpl(EventUtil.java:53)  
          at com.sun.javafx.event.EventUtil.fireEvent(EventUtil.java:33)  
          at javafx.event.Event.fireEvent(Event.java:171)  
          at javafx.scene.Scene$DnDGesture.fireEvent(Scene.java:2631)  
          at javafx.scene.Scene$DnDGesture.processTargetEnterOver(Scene.java:2780)  
          at javafx.scene.Scene$DnDGesture.access$6100(Scene.java:2607)  
          at javafx.scene.Scene$DropTargetListener.dragOver(Scene.java:2534)  
          at com.sun.javafx.tk.quantum.GlassSceneDnDEventHandler$4.run(GlassSceneDnDEventHandler.java:126)  
          at com.sun.javafx.tk.quantum.GlassSceneDnDEventHandler$4.run(GlassSceneDnDEventHandler.java:117)  
          at java.security.AccessController.doPrivileged(Native Method)  
          at com.sun.javafx.tk.quantum.GlassSceneDnDEventHandler.handleDragOver(GlassSceneDnDEventHandler.java:117)  
          at com.sun.javafx.tk.quantum.GlassViewEventHandler.handleDragOver(GlassViewEventHandler.java:422)  
          at com.sun.glass.ui.View.handleDragOver(View.java:701)  
          at com.sun.glass.ui.View.notifyDragOver(View.java:1010)  
          at com.sun.glass.ui.gtk.GtkDnDClipboard.pushToSystemImpl(Native Method)  
          at com.sun.glass.ui.gtk.GtkDnDClipboard.pushToSystem(GtkDnDClipboard.java:24)  
          at com.sun.glass.ui.SystemClipboard.flush(SystemClipboard.java:28)  
          at com.sun.glass.ui.ClipboardAssistance.flush(ClipboardAssistance.java:34)  
          at com.sun.javafx.tk.quantum.QuantumClipboard.flush(QuantumClipboard.java:197)  
          at com.sun.javafx.tk.quantum.QuantumToolkit.startDrag(QuantumToolkit.java:1195)  
          at javafx.scene.Scene$DnDGesture.dragDetectedProcessed(Scene.java:2652)  
          at javafx.scene.Scene$DnDGesture.process(Scene.java:2713)  
          at javafx.scene.Scene$DnDGesture.access$8700(Scene.java:2607)  
          at javafx.scene.Scene$MouseHandler.process(Scene.java:3344)  
          at javafx.scene.Scene$MouseHandler.process(Scene.java:3168)  
          at javafx.scene.Scene$MouseHandler.access$1900(Scene.java:3123)  
          at javafx.scene.Scene.impl_processMouseEvent(Scene.java:1563)  
          at javafx.scene.Scene$ScenePeerListener.mouseEvent(Scene.java:2265)  
          at com.sun.javafx.tk.quantum.GlassViewEventHandler$MouseEventNotification.run(GlassViewEventHandler.java:250)  
          at com.sun.javafx.tk.quantum.GlassViewEventHandler$MouseEventNotification.run(GlassViewEventHandler.java:173)  
          at java.security.AccessController.doPrivileged(Native Method)  
          at com.sun.javafx.tk.quantum.GlassViewEventHandler.handleMouseEvent(GlassViewEventHandler.java:292)  
          at com.sun.glass.ui.View.handleMouseEvent(View.java:528)  
          at com.sun.glass.ui.View.notifyMouse(View.java:922)  
          at com.sun.glass.ui.gtk.GtkApplication._runLoop(Native Method)  
          at com.sun.glass.ui.gtk.GtkApplication$3$1.run(GtkApplication.java:82)  
          at java.lang.Thread.run(Thread.java:724)  
      Exception in thread "JavaFX Application Thread" java.lang.NumberFormatException: null  
          at java.lang.Integer.parseInt(Integer.java:454)  
          at java.lang.Integer.parseInt(Integer.java:527)  
          at com.dx57dc.main.DX57DC$2.handle(DX57DC.java:58)  
          at com.dx57dc.main.DX57DC$2.handle(DX57DC.java:52)  
          at com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:69)  
          at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:217)  
          at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:170)  
          at com.sun.javafx.event.CompositeEventDispatcher.dispatchBubblingEvent(CompositeEventDispatcher.java:38)  
          at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:37)  
          at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:92)  
          at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:35)  
          at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:92)  
          at com.sun.javafx.event.EventUtil.fireEventImpl(EventUtil.java:53)  
          at com.sun.javafx.event.EventUtil.fireEvent(EventUtil.java:33)  
          at javafx.event.Event.fireEvent(Event.java:171)  
          at javafx.scene.Scene$DnDGesture.fireEvent(Scene.java:2631)  
          at javafx.scene.Scene$DnDGesture.processTargetEnterOver(Scene.java:2780)  
          at javafx.scene.Scene$DnDGesture.access$6100(Scene.java:2607)  
          at javafx.scene.Scene$DropTargetListener.dragOver(Scene.java:2534)  
          at com.sun.javafx.tk.quantum.GlassSceneDnDEventHandler$4.run(GlassSceneDnDEventHandler.java:126)  
          at com.sun.javafx.tk.quantum.GlassSceneDnDEventHandler$4.run(GlassSceneDnDEventHandler.java:117)  
          at java.security.AccessController.doPrivileged(Native Method)  
          at com.sun.javafx.tk.quantum.GlassSceneDnDEventHandler.handleDragOver(GlassSceneDnDEventHandler.java:117)  
          at com.sun.javafx.tk.quantum.GlassViewEventHandler.handleDragOver(GlassViewEventHandler.java:422)  
          at com.sun.glass.ui.View.handleDragOver(View.java:701)  
          at com.sun.glass.ui.View.notifyDragOver(View.java:1010)  
          at com.sun.glass.ui.gtk.GtkDnDClipboard.pushToSystemImpl(Native Method)  
          at com.sun.glass.ui.gtk.GtkDnDClipboard.pushToSystem(GtkDnDClipboard.java:24)  
          at com.sun.glass.ui.SystemClipboard.flush(SystemClipboard.java:28)  
          at com.sun.glass.ui.ClipboardAssistance.flush(ClipboardAssistance.java:34)  
          at com.sun.javafx.tk.quantum.QuantumClipboard.flush(QuantumClipboard.java:197)  
          at com.sun.javafx.tk.quantum.QuantumToolkit.startDrag(QuantumToolkit.java:1195)  
          at javafx.scene.Scene$DnDGesture.dragDetectedProcessed(Scene.java:2652)  
          at javafx.scene.Scene$DnDGesture.process(Scene.java:2713)  
          at javafx.scene.Scene$DnDGesture.access$8700(Scene.java:2607)  
          at javafx.scene.Scene$MouseHandler.process(Scene.java:3344)  
          at javafx.scene.Scene$MouseHandler.process(Scene.java:3168)  
          at javafx.scene.Scene$MouseHandler.access$1900(Scene.java:3123)  
          at javafx.scene.Scene.impl_processMouseEvent(Scene.java:1563)  
          at javafx.scene.Scene$ScenePeerListener.mouseEvent(Scene.java:2265)  
          at com.sun.javafx.tk.quantum.GlassViewEventHandler$MouseEventNotification.run(GlassViewEventHandler.java:250)  
          at com.sun.javafx.tk.quantum.GlassViewEventHandler$MouseEventNotification.run(GlassViewEventHandler.java:173)  
          at java.security.AccessController.doPrivileged(Native Method)  
          at com.sun.javafx.tk.quantum.GlassViewEventHandler.handleMouseEvent(GlassViewEventHandler.java:292)  
          at com.sun.glass.ui.View.handleMouseEvent(View.java:528)  
          at com.sun.glass.ui.View.notifyMouse(View.java:922)  
          at com.sun.glass.ui.gtk.GtkApplication._runLoop(Native Method)  
          at com.sun.glass.ui.gtk.GtkApplication$3$1.run(GtkApplication.java:82)  
          at java.lang.Thread.run(Thread.java:724)  
      Exception in thread "JavaFX Application Thread" java.lang.NumberFormatException: null  
          at java.lang.Integer.parseInt(Integer.java:454)  
          at java.lang.Integer.parseInt(Integer.java:527)  
          at com.dx57dc.main.DX57DC$2.handle(DX57DC.java:58)  
          at com.dx57dc.main.DX57DC$2.handle(DX57DC.java:52)  
          at com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:69)  
          at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:217)  
          at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:170)  
          at com.sun.javafx.event.CompositeEventDispatcher.dispatchBubblingEvent(CompositeEventDispatcher.java:38)  
          at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:37)  
          at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:92)  
          at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:35)  
          at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:92)  
          at com.sun.javafx.event.EventUtil.fireEventImpl(EventUtil.java:53)  
          at com.sun.javafx.event.EventUtil.fireEvent(EventUtil.java:33)  
          at javafx.event.Event.fireEvent(Event.java:171)  
          at javafx.scene.Scene$DnDGesture.fireEvent(Scene.java:2631)  
          at javafx.scene.Scene$DnDGesture.processTargetEnterOver(Scene.java:2780)  
          at javafx.scene.Scene$DnDGesture.access$6100(Scene.java:2607)  
          at javafx.scene.Scene$DropTargetListener.dragOver(Scene.java:2534)  
          at com.sun.javafx.tk.quantum.GlassSceneDnDEventHandler$4.run(GlassSceneDnDEventHandler.java:126)  
          at com.sun.javafx.tk.quantum.GlassSceneDnDEventHandler$4.run(GlassSceneDnDEventHandler.java:117)  
          at java.security.AccessController.doPrivileged(Native Method)  
          at com.sun.javafx.tk.quantum.GlassSceneDnDEventHandler.handleDragOver(GlassSceneDnDEventHandler.java:117)  
          at com.sun.javafx.tk.quantum.GlassViewEventHandler.handleDragOver(GlassViewEventHandler.java:422)  
          at com.sun.glass.ui.View.handleDragOver(View.java:701)  
          at com.sun.glass.ui.View.notifyDragOver(View.java:1010)  
          at com.sun.glass.ui.gtk.GtkDnDClipboard.pushToSystemImpl(Native Method)  
          at com.sun.glass.ui.gtk.GtkDnDClipboard.pushToSystem(GtkDnDClipboard.java:24)  
          at com.sun.glass.ui.SystemClipboard.flush(SystemClipboard.java:28)  
          at com.sun.glass.ui.ClipboardAssistance.flush(ClipboardAssistance.java:34)  
          at com.sun.javafx.tk.quantum.QuantumClipboard.flush(QuantumClipboard.java:197)  
          at com.sun.javafx.tk.quantum.QuantumToolkit.startDrag(QuantumToolkit.java:1195)  
          at javafx.scene.Scene$DnDGesture.dragDetectedProcessed(Scene.java:2652)  
          at javafx.scene.Scene$DnDGesture.process(Scene.java:2713)  
          at javafx.scene.Scene$DnDGesture.access$8700(Scene.java:2607)  
          at javafx.scene.Scene$MouseHandler.process(Scene.java:3344)  
          at javafx.scene.Scene$MouseHandler.process(Scene.java:3168)  
          at javafx.scene.Scene$MouseHandler.access$1900(Scene.java:3123)  
          at javafx.scene.Scene.impl_processMouseEvent(Scene.java:1563)  
          at javafx.scene.Scene$ScenePeerListener.mouseEvent(Scene.java:2265)  
          at com.sun.javafx.tk.quantum.GlassViewEventHandler$MouseEventNotification.run(GlassViewEventHandler.java:250)  
          at com.sun.javafx.tk.quantum.GlassViewEventHandler$MouseEventNotification.run(GlassViewEventHandler.java:173)  
          at java.security.AccessController.doPrivileged(Native Method)  
          at com.sun.javafx.tk.quantum.GlassViewEventHandler.handleMouseEvent(GlassViewEventHandler.java:292)  
          at com.sun.glass.ui.View.handleMouseEvent(View.java:528)  
          at com.sun.glass.ui.View.notifyMouse(View.java:922)  
          at com.sun.glass.ui.gtk.GtkApplication._runLoop(Native Method)  
          at com.sun.glass.ui.gtk.GtkApplication$3$1.run(GtkApplication.java:82)  
          at java.lang.Thread.run(Thread.java:724)  
      Exception in thread "JavaFX Application Thread" java.lang.NumberFormatException: null  
          at java.lang.Integer.parseInt(Integer.java:454)  
          at java.lang.Integer.parseInt(Integer.java:527)  
          at com.dx57dc.main.DX57DC$2.handle(DX57DC.java:58)  
          at com.dx57dc.main.DX57DC$2.handle(DX57DC.java:52)  
          at com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:69)  
          at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:217)  
          at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:170)  
          at com.sun.javafx.event.CompositeEventDispatcher.dispatchBubblingEvent(CompositeEventDispatcher.java:38)  
          at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:37)  
          at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:92)  
          at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:35)  
          at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:92)  
          at com.sun.javafx.event.EventUtil.fireEventImpl(EventUtil.java:53)  
          at com.sun.javafx.event.EventUtil.fireEvent(EventUtil.java:33)  
          at javafx.event.Event.fireEvent(Event.java:171)  
          at javafx.scene.Scene$DnDGesture.fireEvent(Scene.java:2631)  
          at javafx.scene.Scene$DnDGesture.processTargetEnterOver(Scene.java:2780)  
          at javafx.scene.Scene$DnDGesture.access$6100(Scene.java:2607)  
          at javafx.scene.Scene$DropTargetListener.dragOver(Scene.java:2534)  
          at com.sun.javafx.tk.quantum.GlassSceneDnDEventHandler$4.run(GlassSceneDnDEventHandler.java:126)  
          at com.sun.javafx.tk.quantum.GlassSceneDnDEventHandler$4.run(GlassSceneDnDEventHandler.java:117)  
          at java.security.AccessController.doPrivileged(Native Method)  
          at com.sun.javafx.tk.quantum.GlassSceneDnDEventHandler.handleDragOver(GlassSceneDnDEventHandler.java:117)  
          at com.sun.javafx.tk.quantum.GlassViewEventHandler.handleDragOver(GlassViewEventHandler.java:422)  
          at com.sun.glass.ui.View.handleDragOver(View.java:701)  
          at com.sun.glass.ui.View.notifyDragOver(View.java:1010)  
          at com.sun.glass.ui.gtk.GtkDnDClipboard.pushToSystemImpl(Native Method)  
          at com.sun.glass.ui.gtk.GtkDnDClipboard.pushToSystem(GtkDnDClipboard.java:24)  
          at com.sun.glass.ui.SystemClipboard.flush(SystemClipboard.java:28)  
          at com.sun.glass.ui.ClipboardAssistance.flush(ClipboardAssistance.java:34)  
          at com.sun.javafx.tk.quantum.QuantumClipboard.flush(QuantumClipboard.java:197)  
          at com.sun.javafx.tk.quantum.QuantumToolkit.startDrag(QuantumToolkit.java:1195)  
          at javafx.scene.Scene$DnDGesture.dragDetectedProcessed(Scene.java:2652)  
          at javafx.scene.Scene$DnDGesture.process(Scene.java:2713)  
          at javafx.scene.Scene$DnDGesture.access$8700(Scene.java:2607)  
          at javafx.scene.Scene$MouseHandler.process(Scene.java:3344)  
          at javafx.scene.Scene$MouseHandler.process(Scene.java:3168)  
          at javafx.scene.Scene$MouseHandler.access$1900(Scene.java:3123)  
          at javafx.scene.Scene.impl_processMouseEvent(Scene.java:1563)  
          at javafx.scene.Scene$ScenePeerListener.mouseEvent(Scene.java:2265)  
          at com.sun.javafx.tk.quantum.GlassViewEventHandler$MouseEventNotification.run(GlassViewEventHandler.java:250)  
          at com.sun.javafx.tk.quantum.GlassViewEventHandler$MouseEventNotification.run(GlassViewEventHandler.java:173)  
          at java.security.AccessController.doPrivileged(Native Method)  
          at com.sun.javafx.tk.quantum.GlassViewEventHandler.handleMouseEvent(GlassViewEventHandler.java:292)  
          at com.sun.glass.ui.View.handleMouseEvent(View.java:528)  
          at com.sun.glass.ui.View.notifyMouse(View.java:922)  
          at com.sun.glass.ui.gtk.GtkApplication._runLoop(Native Method)  
          at com.sun.glass.ui.gtk.GtkApplication$3$1.run(GtkApplication.java:82)  
          at java.lang.Thread.run(Thread.java:724)  
      Exception in thread "JavaFX Application Thread" java.lang.NumberFormatException: null  
          at java.lang.Integer.parseInt(Integer.java:454)  
          at java.lang.Integer.parseInt(Integer.java:527)  
          at com.dx57dc.main.DX57DC$2.handle(DX57DC.java:58)  
          at com.dx57dc.main.DX57DC$2.handle(DX57DC.java:52)  
          at com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:69)  
          at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:217)  
          at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:170)  
          at com.sun.javafx.event.CompositeEventDispatcher.dispatchBubblingEvent(CompositeEventDispatcher.java:38)  
          at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:37)  
          at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:92)  
          at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:35)  
          at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:92)  
          at com.sun.javafx.event.EventUtil.fireEventImpl(EventUtil.java:53)  
          at com.sun.javafx.event.EventUtil.fireEvent(EventUtil.java:33)  
          at javafx.event.Event.fireEvent(Event.java:171)  
          at javafx.scene.Scene$DnDGesture.fireEvent(Scene.java:2631)  
          at javafx.scene.Scene$DnDGesture.processTargetEnterOver(Scene.java:2780)  
          at javafx.scene.Scene$DnDGesture.access$6100(Scene.java:2607)  
          at javafx.scene.Scene$DropTargetListener.dragOver(Scene.java:2534)  
          at com.sun.javafx.tk.quantum.GlassSceneDnDEventHandler$4.run(GlassSceneDnDEventHandler.java:126)  
          at com.sun.javafx.tk.quantum.GlassSceneDnDEventHandler$4.run(GlassSceneDnDEventHandler.java:117)  
          at java.security.AccessController.doPrivileged(Native Method)  
          at com.sun.javafx.tk.quantum.GlassSceneDnDEventHandler.handleDragOver(GlassSceneDnDEventHandler.java:117)  
          at com.sun.javafx.tk.quantum.GlassViewEventHandler.handleDragOver(GlassViewEventHandler.java:422)  
          at com.sun.glass.ui.View.handleDragOver(View.java:701)  
          at com.sun.glass.ui.View.notifyDragOver(View.java:1010)  
          at com.sun.glass.ui.gtk.GtkDnDClipboard.pushToSystemImpl(Native Method)  
          at com.sun.glass.ui.gtk.GtkDnDClipboard.pushToSystem(GtkDnDClipboard.java:24)  
          at com.sun.glass.ui.SystemClipboard.flush(SystemClipboard.java:28)  
  
  ................................................................
  </code></pre>
  
  <p>(and more)</p>
  
  <p>The content of <code>int incomingIndex = Integer.parseInt(dragboard.getString());</code> is Null</p>
  
  <p>Can you tell me how I can fix this issue or rewrite the code?</p>