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I opened an editor on sun-web.xml. Then I cloned it 3 times, and selected "close all other windows" from the popup window of the 3rd clone (4th editor). There may have been one more step, but I can't remember it now. Anyway, I got the attached trace. Further investigation suggests that cloned editors may not have a dataobject attached (bad).
Created attachment 29650 [details] stack trace
Looks like it was cloning one of the cloned editors that produce the NPE (which make sense if a cloned editor somehow has a null dataobject because that's what causes the NPE during initialization of the new editor).
To reproduce: Open config editor. Clone once. Close clone. Clone original again - BANG.
Problem was that the dataobject was caching a the first instance of the editor, so when a second editor notified the dataobject it was being close, the dataobject called a reset method on the first editor (which was staying open) rather than the second one that was being closed. Fixed by removing caching of the top component by the dataobject (and update internals accordingly) and having editor pass reference to self to dataobject on close notification.