This bug was originally marked as duplicate of bug 186297, that is already resolved. This bug is still valid, so this seems to be another bug, but it might be related.

Build: NetBeans IDE 6.9 (Build 201006101454)
VM: Java HotSpot(TM) 64-Bit Server VM, 16.3-b01, Java(TM) SE Runtime Environment, 1.6.0_20-b02
OS: Windows 7

User Comments:
GUEST: simply stopped debugging now -.-

GUEST: Debuging web application

GUEST: Noticed this after debugging my program

GUEST: trying to debug .. debug failing to stop at breakpoints

GUEST: I was debugging and clicked the "STOP"-button when this error occured

mmirilovic: Switch to Loaded classes

GUEST: Debugging to wrong remote host.

artisan: This happens once in while during testing. 

Don't know to reproduce.

GUEST: debugging

artisan: running a unit test

narve: Debugging my Swing GUI

mmirilovic: Steps to reproduce:
- I had opened MemoryView project
- CLass breakpoint in group 'xxxx' and another line breakpoint inside constructor
- started debugger
- slected Breakpoint's group node 'xxx' and pushed 'Delete All' from context menu
- continue in debugging

GUEST: was viewing variables and then stopped debugging

phill84: deploying a webapp in tomcat

milinkp: I dont know when its comes.. Its happened when i try to execute the program ,

GUEST: stopped debugging

GUEST: Stop debug program and see problem.

GUEST: Don't know exactly when the problem occured.




Stacktrace: 
java.lang.NullPointerException: Null child at index 0, parent: Root, model: org.netbeans.spi.viewmodel.Models$CompoundModel@17fde781
  TreeModel = org.netbeans.spi.viewmodel.Models$CompoundTreeModel@4e7a84f7
    org.netbeans.modules.debugger.jpda.projects.ToolTipTreeModelFilter@583eceb8
      org.netbeans.modules.debugger.jpda.ui.models.VariablesTreeModelFilter@254ef3d9
        org.netbeans.modules.debugger.jpda.ui.models.VariablesTreeModelFilterSI@626b3eb1
          org.netbeans.spi.viewmodel.Models$DelegatingTreeModel@1e69d275
    org.netbeans.modules.debugger.jpda.models.LocalsTreeModel@754dfe86
  NodeModel = org.netbeans.spi.viewmodel.Models$CompoundNodeModel@4443cc1f
    org.netbeans.modules.debugger.jpda.projects.ToolTipNodeModelFilter@375bf4b
      org.netbeans.modules.debugger.jpda.ui.models.VariablesTreeModelFilter@254ef3d9
        org.netbeans.modules.debugger.jpda.ui.models.VariablesNodeModel@139e703d
        org.netbeans.modules.debugger.jpda.ui.models.VariablesTreeModelFilterSI@626b3eb1
  TableMo
   at org.netbeans.modules.viewmodel.TreeModelNode$TreeModelChildren.applyChildren(TreeModelNode.java:1380)
   at org.netbeans.modules.viewmodel.TreeModelNode$TreeModelChildren.refreshLazyChildren(TreeModelNode.java:1356)
   at org.netbeans.modules.viewmodel.TreeModelNode$TreeModelChildren.refreshChildren(TreeModelNode.java:1216)
   at org.netbeans.modules.viewmodel.TreeModelNode.refreshTheChildren(TreeModelNode.java:823)
   at org.netbeans.modules.viewmodel.TreeModelNode.refreshTheChildren(TreeModelNode.java:806)
   at org.netbeans.modules.viewmodel.TreeModelNode$5.run(TreeModelNode.java:549)