Exception happens in the  restoreComponentState method in "javax/swing/plaf/basic/BasicTreeUI.java"

//Following is a 1.4.2_04 based source snapshot where NPE happens. 
         /** 
           * called to restore the state of a component  
           * because a drop was not performed. 
          */ 
          protected void restoreComponentState(JComponent comp) { 
              JTree tree = (JTree)comp; 
              tree.setSelectionRows(selectedIndices);  <-- NPE happens here,since tree is not checked for Null 
          }

Tested env. Java Studio Enterprise Build 050713_14, winxp, jdk1.5.0_03
http://balui.sfbay/kits/oriondev/main/Build050713_14/installers/jstudio_ent8_ea-win.exe
Steps :
0. Install JSE from above location, then open JSE IDE from 
<install location>/ide/bin/runide.exe
1. Create a platform independent UML project (File-> New Project)
2. create a class diagram in the UML project
3. Bring up design center window using the menu
   "Window->UML Modelling->Design Center"
4. Select any of the  design center nodes and try to drag it into the
   almostempty area above the tree, that just has a funnel and a refresh icons

you will see an exception in the console with following stack trace :

java.lang.NullPointerException
        at javax.swing.plaf.basic.BasicTreeUI$TreeDropTargetListener.restoreComp
onentState(BasicTreeUI.java:3075)
        at javax.swing.plaf.basic.BasicDropTargetListener.dragExit(BasicDropTarg
etListener.java:252)
        at javax.swing.TransferHandler$SwingDropTarget.dragExit(TransferHandler.
java:601)
        at sun.awt.dnd.SunDropTargetContextPeer.processExitMessage(SunDropTarget
ContextPeer.java:366)
        at sun.awt.dnd.SunDropTargetContextPeer.access$700(SunDropTargetContextP
eer.java:53)
        at sun.awt.dnd.SunDropTargetContextPeer$EventDispatcher.dispatchExitEven
t(SunDropTargetContextPeer.java:799)
        at sun.awt.dnd.SunDropTargetContextPeer$EventDispatcher.dispatchEvent(Su
nDropTargetContextPeer.java:733)
        at sun.awt.dnd.SunDropTargetEvent.dispatch(SunDropTargetEvent.java:29)
        at java.awt.Component.dispatchEventImpl(Component.java:3826)
        at java.awt.Container.dispatchEventImpl(Container.java:2024)
        at java.awt.Component.dispatchEvent(Component.java:3803)
        at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4212
)
        at java.awt.LightweightDispatcher.trackMouseEnterExit(Container.java:400
9)
        at java.awt.LightweightDispatcher.processDropTargetEvent(Container.java:
3955)
        at java.awt.LightweightDispatcher.dispatchEvent(Container.java:3817)
        at java.awt.Container.dispatchEventImpl(Container.java:2010)
        at java.awt.Window.dispatchEventImpl(Window.java:1774)
        at java.awt.Component.dispatchEvent(Component.java:3803)
        at java.awt.EventQueue.dispatchEvent(EventQueue.java:463)
        at java.awt.EventDispatchThread.pumpOneEventForHierarchy(EventDispatchTh
read.java:242)
        at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThre
ad.java:163)
        at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:157)

        at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:149)

        at java.awt.EventDispatchThread.run(EventDispatchThread.java:110)