title: "subject" parameter in "externalObjectDrop" function is empty
body: <p>e.subject parameter in the "externalObjectDrop" function comes with count=0,
  although the dragged parts are moved to the model.</p>
  
  <p>in the diagram:</p>
  
  <pre><code>  allowCopy: true,
    allowUndo: false,
    allowInsert: true,
    allowDragOut: true,
    allowDrop: true,
    allowMove: true,
    "ExternalObjectsDropped": (e) =&gt; {
                  this.externalObjectDrop(e);
              }
  </code></pre>
  
  <p>the function: </p>
  
  <pre><code>  externalObjectDrop(e) {
   e.diagram.commandHandler.deleteSelection();
   e.parameter.commandHandler.deleteSelection();
     //e.subject.count is 0.
  }
  </code></pre>