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i think, i reported this error some time ago, but my search didn't find anything, so i post it "again"... i have 3 projects. the first has shared icons and the other 2 projects use this icons. when i create a action inside one of the 2 projects and assign them a icon of project 1 (the shared icons) then i can see the icon inside the designer, but inside the property file of the form there you can read xxxaction.icon={$null} and the next time i open the form inside the designer, the icon is gone! if i do the same with an icon of a button ("outside" an action) then it works.
I was able to reproduce this issue. Despite the reporter's note it doesn't seem to work even when the icon is set directly (not through action). The problem is in DesignResourceMap, it is not able to handle such icons. It uses class loader from SOURCE ClassPath. It should use something like ClassPathUtils.getProjectClassLoader(srcFile); We also should make sure that this classloader is correctly updated when project's classpath is updated.
NetBeans support of Swing Application Framework has been discontinued => closing this issue as WONTFIX.