This case very much reproducable.

How to generate this NPE
1) Start a jconsole
2) connect to any jconsole application (or any java application).
3) after sucessful connection disconnect and browse tabs (Mbean).
4) Mbean tab shows a Tree of Mbeans registered.
5) try to open on any of these Mbeans
6) Displays a error message 'The client has been closed'
7) and throws this NPE.
* after this NPE it doesn;t throw any eroror or it will respond for other mbeans only for first.
* connect back with the latest functionality, still you don;t be able to browse throw the mbeans in Mbeans tab.



Exception in thread "Worker-MBeans-5688" java.lang.NullPointerException
        at java.awt.Container.addImpl(Container.java:1011)
        at java.awt.Container.add(Container.java:897)
        at sun.tools.jconsole.inspector.XSheet.updatePanel(XSheet.java:272)
        at sun.tools.jconsole.inspector.XSheet.access$500(XSheet.java:37)
        at sun.tools.jconsole.inspector.XSheet$1.run(XSheet.java:262)
        at sun.tools.jconsole.Worker.run(Worker.java:37)