In the "Objects" panel of the ObjectBrowser:

If you toggle buttons that display/hide classes/interfaces, you have to wait
pretty long time (maybe 10 seconds) to see results. After some changes
(toggles) the Objects panel may stop responding (it does not display any
objects). If you close the Object Browser after the Object Browser stops
displaying objects and try to open it (from the toolbar) again, you get the
following exception:


Exception occurred during event dispatching:
java.lang.IllegalStateException: Cannot initialize 0 th node of node
org.openide.nodes.AbstractNode@db1b062a[Name=null, displayName=null, handle=null]
it already belongs to node com.netbeans.developer.impl.PackageNode@4d430639
[Name=Repository, displayName=Repository, handle=null]
        at org.openide.nodes.Children.throwAlreadyUsedException(Children.java:582)
        at org.openide.nodes.Children$6.run(Children.java:565)
        at org.openide.util.Mutex.doEvent(Mutex.java:455)
        at org.openide.util.Mutex.writeAccess(Mutex.java:240)
        at org.openide.nodes.Children.initialize(Children.java:555)
        at org.openide.nodes.Children$5.run(Children.java:532)
        at org.openide.util.Mutex.doEventAccess(Mutex.java:480)
        at org.openide.util.Mutex.doReadAccess(Mutex.java:165)
        at org.openide.util.Mutex.readAccess(Mutex.java:104)
        at org.openide.nodes.Children.getNodes(Children.java:528)
        at org.openide.explorer.view.ListView.updateList(ListView.java:488)
        at org.openide.explorer.view.ListView.access$8(ListView.java:477)
        at org.openide.explorer.view.ListView$9.run(ListView.java:412)
        at org.openide.util.Mutex.doEvent(Mutex.java:455)
        at org.openide.util.Mutex.readAccess(Mutex.java:146)
        at org.openide.explorer.view.ListView.addNotify(ListView.java:402)