Steps to reproduce:
1) create a memory representation of ClassElement
ClassElement.forName(some_class_that_have_inner_classes)
2) use it
new ClassElementNode(element, new ClassChildren(element), false)

the easiest way how to reproduce it is to create an RMI service, add an inner class to the interface,
register it to the RMI Registry and then browse the registry using RMI Registry Browser (Runtime | 
RMI Registry). 


The exception:

Wed Aug 29 18:55:58 GMT+02:00 2001: java.lang.NullPointerException: null
java.lang.NullPointerException
        at org.netbeans.modules.clazz.ClassElementImpl.createInnersMap(ClassElementImpl.java:321)
        at org.netbeans.modules.clazz.ClassElementImpl.getClasses(ClassElementImpl.java:217)
        at org.openide.src.ClassElement.getClasses(ClassElement.java:626)
        at org.openide.src.nodes.ClassChildren.getKeysOfType(ClassChildren.java:233)
        at org.openide.src.nodes.ClassChildren.refreshKeys(ClassChildren.java:204)
        at org.openide.src.nodes.ClassChildren.refreshAllKeys(ClassChildren.java:193)
        at org.openide.src.nodes.ClassChildren.addNotify(ClassChildren.java:147)
        at org.openide.nodes.Children.callAddNotify(Children.java:336)
        at org.openide.nodes.ChildrenArray.create(ChildrenArray.java:51)
        at org.openide.nodes.Children.getArray(Children.java:365)
        at org.openide.nodes.Children.computeNodes(Children.java:355)
        at org.openide.nodes.Children.getNodes(Children.java:284)
        at org.openide.explorer.view.VisualizerNode$1.run(VisualizerNode.java:143)
        at org.openide.util.Mutex.readAccess(Mutex.java:176)
        at org.openide.explorer.view.VisualizerNode.getChildren(VisualizerNode.java:141)
        at org.openide.explorer.view.VisualizerNode.getChildCount(VisualizerNode.java:181)
        at javax.swing.tree.DefaultTreeModel.getChildCount(DefaultTreeModel.java:163)
        at javax.swing.tree.VariableHeightLayoutCache$TreeStateNode.expand(VariableHeightLayoutCache.java:1412)
        at javax.swing.tree.VariableHeightLayoutCache$TreeStateNode.expand(VariableHeightLayoutCache.java:1221)
        at javax.swing.tree.VariableHeightLayoutCache.ensurePathIsExpanded(VariableHeightLayoutCache.java:917)
        at javax.swing.tree.VariableHeightLayoutCache.setExpandedState(VariableHeightLayoutCache.java:162)
        at javax.swing.plaf.basic.BasicTreeUI.updateExpandedDescendants(BasicTreeUI.java:1428)
        at javax.swing.plaf.basic.BasicTreeUI$TreeExpansionHandler.treeExpanded(BasicTreeUI.java:2239)
        at javax.swing.JTree.fireTreeExpanded(JTree.java:2114)
        at javax.swing.JTree.setExpandedState(JTree.java:2794)
        at javax.swing.JTree.expandPath(JTree.java:1646)
        at javax.swing.plaf.basic.BasicTreeUI.toggleExpandState(BasicTreeUI.java:2022)
        at javax.swing.plaf.basic.BasicTreeUI.handleExpandControlClick(BasicTreeUI.java:2009)
        at javax.swing.plaf.basic.BasicTreeUI.checkForClickInExpandControl(BasicTreeUI.java:1963)
        at javax.swing.plaf.basic.BasicTreeUI$MouseHandler.mousePressed(BasicTreeUI.java:2674)
        at java.awt.AWTEventMulticaster.mousePressed(AWTEventMulticaster.java:220)
        at java.awt.AWTEventMulticaster.mousePressed(AWTEventMulticaster.java:220)
        at java.awt.AWTEventMulticaster.mousePressed(AWTEventMulticaster.java:220)
        at java.awt.Component.processMouseEvent(Component.java:3714)
        at java.awt.Component.processEvent(Component.java:3546)
        at java.awt.Container.processEvent(Container.java:1164)
        at java.awt.Component.dispatchEventImpl(Component.java:2595)
        at java.awt.Container.dispatchEventImpl(Container.java:1213)
        at java.awt.Component.dispatchEvent(Component.java:2499)
        at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:2451)
[catch] at java.awt.LightweightDispatcher.processMouseEvent(Container.java:2210)
        at java.awt.LightweightDispatcher.dispatchEvent(Container.java:2125)
        at java.awt.Container.dispatchEventImpl(Container.java:1200)
        at java.awt.Window.dispatchEventImpl(Window.java:912)
        at java.awt.Component.dispatchEvent(Component.java:2499)
        at java.awt.EventQueue.dispatchEvent(EventQueue.java:319)
        at java.awt.EventDispatchThread.pumpOneEvent(EventDispatchThread.java:103)
        at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:93)
        at java.awt.EventDispatchThread.run(EventDispatchThread.java:84)