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I was using ctrl+w to close several document windows. After closing about 3 documents I got this exception.
Created attachment 37791 [details] log
It might not be a problem in openide/text, but that's where the exception came from.
This was a race condition in the PositionRef$Manager, which is really not preprared for threading...
NPE is now prevented in NbDocument, rev. 1.7 - in such case the method returns 0