title: in eclipse: NullPointerException at org.eclipse.e4.ui.workbench.addons.dndaddon.DragAgent.dragFinished(DragAgent.java:171)
body: <p>While drag and drop file tab to reorder open files of different perspective we are getting NullPointer error in DND code</p>
  
  <p>java.lang.NullPointerException
      at org.eclipse.e4.ui.workbench.addons.dndaddon.DragAgent.dragFinished(DragAgent.java:171)
      at org.eclipse.e4.ui.workbench.addons.dndaddon.PartDragAgent.dragFinished(PartDragAgent.java:93)
      at org.eclipse.e4.ui.workbench.addons.dndaddon.DnDManager.finishDrag(DnDManager.java:276)
      at org.eclipse.e4.ui.workbench.addons.dndaddon.DnDManager.startDrag(DnDManager.java:243)
      at org.eclipse.e4.ui.workbench.addons.dndaddon.DnDManager$1.dragDetected(DnDManager.java:87)
      at org.eclipse.swt.widgets.TypedListener.handleEvent(TypedListener.java:127)
      at org.eclipse.swt.widgets.EventTable.sendEvent(EventTable.java:84)
      at org.eclipse.swt.widgets.Display.sendEvent(Display.java:4362)
      at org.eclipse.swt.widgets.Widget.sendEvent(Widget.java:1113)
      at org.eclipse.swt.widgets.Display.runDeferredEvents(Display.java:4180)
      at org.eclipse.swt.widgets.Display.readAndDispatch(Display.java:3769)</p>