title: WritablewWorkbook.write() in jxl produces excel file which is empty 0b
body: <p>What am I doing wrong here?  The values of the JTextFields are valid and the code works to print the results to the Console. I get a file in my workspace, but its size = 0b and displays an empty spreadsheet.</p>
  
  <pre><code>import java.awt.event.ActionEvent;
  import java.awt.event.ActionListener;
  import java.io.File;
  import java.io.IOException;
  
  import javax.swing.JTextField;
  
  import jxl.Cell;
  import jxl.Sheet;
  import jxl.Workbook;
  import jxl.WorkbookSettings;
  import jxl.format.CellFormat;
  import jxl.read.biff.BiffException;
  import jxl.write.Label;
  import jxl.write.WritableCellFormat;
  import jxl.write.WritableSheet;
  import jxl.write.WritableWorkbook;
  import jxl.write.WriteException;
  import jxl.write.biff.RowsExceededException;
  
  public class convertFileAction implements ActionListener {
      File _file;
      JTextField _filePath;
      JTextField _whichColumn;
      String _path;
      int _column;
  
      public convertFileAction(JTextField tfFilePath, JTextField tfWhichColumn) {
          _filePath = tfFilePath;
          _whichColumn = tfWhichColumn;
      }
  
      @Override
      public void actionPerformed(ActionEvent e) {
          _path = _filePath.getText();
          _file = new File(_path);
          _column = Integer.parseInt(_whichColumn.getText());
          _column = _column - 1;
  
          try {
              WorkbookSettings wbSettings = new WorkbookSettings();
              wbSettings.setUseTemporaryFileDuringWrite(true);
              Workbook workbook = Workbook.getWorkbook(_file);
              // System.out.println(""+_file.exists());
              Sheet sheet = workbook.getSheet(0);
              WritableWorkbook genWorkbook = Workbook.createWorkbook(new File(
                      "output.xls"));
              WritableSheet genSheet = genWorkbook.createSheet("Ad IDs", 0);
              Label headerIc = new Label(1, 1, "ifContexts");
              genSheet.addCell(headerIc);
              for (int i = 0; sheet.getCell(_column, i) != null; i++) {
                  Cell c = sheet.getCell(_column, i);
                      CellFormat readFormat = c.getCellFormat();
                      WritableCellFormat writeFormat = new WritableCellFormat(
                              readFormat);
                      System.out.println(c.getContents());
                      String ifContext = c.getContents().toString();
                      Label labIfContext = new Label(1, i + 1, ifContext);
                      labIfContext.setCellFormat(writeFormat);
                      genSheet.addCell(labIfContext);
              }
              genWorkbook.write();
              workbook.close();
              genWorkbook.close();
  
          } catch (BiffException | IOException e1) {
              // TODO Auto-generated catch block
              e1.printStackTrace();
          } catch (RowsExceededException e1) {
              // TODO Auto-generated catch block
              e1.printStackTrace();
          } catch (WriteException e1) {
              // TODO Auto-generated catch block
              e1.printStackTrace();
          }
  
      }
  
  }
  </code></pre>
  
  <p>I do receive an error in the Console, but I'm not sure if it's related.  Here it is anyways:</p>
  
  <pre><code>Exception in thread "AWT-EventQueue-1" jxl.common.AssertionFailed
      at jxl.common.Assert.verify(Assert.java:37)
      at jxl.biff.XFRecord.&lt;init&gt;(XFRecord.java:521)
      at jxl.write.biff.CellXFRecord.&lt;init&gt;(CellXFRecord.java:69)
      at jxl.write.WritableCellFormat.&lt;init&gt;(WritableCellFormat.java:96)
      at convertFileAction.actionPerformed(convertFileAction.java:54)
      at java.awt.Button.processActionEvent(Unknown Source)
      at java.awt.Button.processEvent(Unknown Source)
      at java.awt.Component.dispatchEventImpl(Unknown Source)
      at java.awt.Component.dispatchEvent(Unknown Source)
      at java.awt.EventQueue.dispatchEventImpl(Unknown Source)
      at java.awt.EventQueue.access$200(Unknown Source)
      at java.awt.EventQueue$3.run(Unknown Source)
      at java.awt.EventQueue$3.run(Unknown Source)
      at java.security.AccessController.doPrivileged(Native Method)
      at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
      at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
      at java.awt.EventQueue$4.run(Unknown Source)
      at java.awt.EventQueue$4.run(Unknown Source)
      at java.security.AccessController.doPrivileged(Native Method)
      at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
      at java.awt.EventQueue.dispatchEvent(Unknown Source)
      at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
      at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
      at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
      at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
      at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
      at java.awt.EventDispatchThread.run(Unknown Source)
  </code></pre>
  
  <p>Much appreciated.</p>