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I see a problem when using the SwingX JXTaskPaneContainer and JXTaskPane components. This is reproducible by creating a simple form with a JXTaskPaneContainer and a couple JXTaskPanes inside it. I also put a few JButtons or JXHyperlinks. Save this form (test it to prove to yourself that it works). A this point, the only error should be an annoying warning when constructing the form. It complains that there is a null layout. Save the project and all files and then exit NetBeans. Start netbeans again, and you'll see a bunch of errors about the form when starting concerning the layout. This is with the SwingX 051211 release.
Created attachment 27809 [details] messages log
I also tried the NB 5.1 release on Windows... this had problems which were worse.
JXTaskPane and JXTaskPaneContainer have a special layout which form editor handles as "unknown" - thus the warning. Two problems are connected with this case: - the unknown layout is reported as error when loading a form - components in container with unknown layout are not loaded correctly The first problem is harmless, though it looks quite strange (user is warned about errors in form, but can ignore it). The second problem (not originally reported here) makes the loaded container unusable - to reproduce add a component (e.g. JXTaskPane) to JXTaskPaneContainer and after reloading try to delete it. An exception occurs since the components are not correctly loaded into the "unknown" layout. I've fixed both problems in trunk. The warning is now just silently logged, and components are loaded as expected. Diff is here: http://form.netbeans.org/servlets/ReadMsg?list=cvs&msgNo=3481
See also issue 70683 (suggesting further improvement for future).
I have reviewed the fix - it seems to be correct.
Fix integrated also into 5.0.