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the cause of this one is within:
com.extjs.gxt.ui.client.widget.treepanel.getContainer(TreeNode)
line 87, because node.getElement() might be null!

called by...

after refreshing a tree grid by calling refresh(headerToo) the joint elements within TreeGrid.TreeNode are not reset to null, so that they will no longer be children of there newly created parents....

Thanks G.edwin,
I got it working this way, the problem I still had was in web mode in combination with tomcat. Since it is not easy to control the Classloader in tomcat the gwt-servlet.jar was...