Make sure a notification area is available on the gnome panel. Run the attached test. You will see a frame with many buttons and a tray icon will be added to the notification area. Click on 'set image' button. A file dialog will get opened. Leave it open. Click on the tray icon, drag it and drop it inside the FileDialog. You will see a flurry of NullPointer Exceptions being thrown on the console.

This is reproducible only on XToolkit atleast since JDK6-b59d. Not reproducible on Win32.

Here is the stack trace -
--------------------------
java.lang.NullPointerException
at java.awt.LightweightDispatcher.eventDispatched(Container.java:4077)
at java.awt.Toolkit$SelectiveAWTEventListener.eventDispatched(Toolkit.java:2339)
at java.awt.Toolkit.notifyAWTEventListeners(Toolkit.java:2190)
at java.awt.TrayIcon.dispatchEvent(TrayIcon.java:665)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:604)
at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:273)
at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:183)
at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:177)
at java.awt.Dialog$1.run(Dialog.java:1032)
at java.awt.Dialog$2.run(Dialog.java:1078)
at java.security.AccessController.doPrivileged(Native Method)
at java.awt.Dialog.show(Dialog.java:1076)
at java.awt.Component.show(Component.java:1406)
at java.awt.Component.setVisible(Component.java:1359)
at java.awt.Window.setVisible(Window.java:695)
at java.awt.Dialog.setVisible(Dialog.java:972)
at SystemTrayTest$ActionHandler.actionPerformed(SystemTrayTest.java:217)at java.awt.Button.processActionEvent(Button.java:388)
at java.awt.Button.processEvent(Button.java:356)
at java.awt.Component.dispatchEventImpl(Component.java:4365)
at java.awt.Component.dispatchEvent(Component.java:4195)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:599)
at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:273)
at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:183)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:173)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:168)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:160)
at java.awt.EventDispatchThread.run(EventDispatchThread.java:121)