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Open any Maven pom multiproject. I did https://hg.netbeans.org/html4j/ Find "Modules" node and click "Create New Module..." choose Maven/Java Application. The new module is created, but it's source packages aren't accessible. Restart helps to associate the source package properly.
If others can reproduce the problem, then I believe it is a high priority regression.
*** This bug has been marked as a duplicate of bug 256175 ***