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I'm not sure if this is for the windowing manager or refactoring (due to presence in stack trace) or somewhere else. It might even just be a JDK bug (I'm running on JDK 1.6.0 b90) or at least a JDK dependent bug. Anyway, I got the attached exception attempting to click the close button on the last of a row of 4 "Find Usages" tabs in the Find Usages pane. I've seen it before over the past several weeks. This is a relatively current dev build of 5.5 (code from around July 17, 2006). Once this appears, you pretty much have to restart the IDE as there is no way to close any of the Find Usages panes. Clicking on any one of them will reproduce the exception over and over.
Created attachment 31978 [details] stack trace
Reassigning to "refactoring" for evaluation.
Cannot reproduce
I'm sorry I cannot reproduce it. It is probably JDK bug, which was recently fixed.