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This is unreproducible issue. It happened to me when I pressed "Add >" button in Contents editor dialog. (Before this I've click on "Remove All <<" button). When I cancelled Contents editor dialog, opened again and performed mentioned actions again, exception was shown again. When I created new JAR Recipe and tried previous steps to reproduce issue, no exception has occured. Even if I tried to reproduce it on previous JAR Recipe.
From looking at the code, I don't see what could be the problem. As it is not reproducible it is not possible to be tracked down. I'm closing it - please reopen when it appears again.