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I had .../nb_all/core open and opened a lot of sources from it, incl. a few form files. (Code tab only; did not touch design tab.) Closed core project. All regular *.java were closed but form sources stayed open.
Problem is in MultiViewCloneableTopComponent which doesn't return any activated node nor any DataObject in its lookup (issue 43830), after fix this issue it'll works with both java files and form files.
Blocking issue 43830 has been fixed, now works for me for form files.
This issue was solved long time ago. Because nobody has reopened it neither added comments, we are verifying/closing it now. If you are still able to reproduce the problem, please reopen. Thanks in advance.