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Stando, your change 1df8daf75020 in ImageUtilities broke org.openide.awt.MenuBarNotInAWTTest. In fact I believe your change violates Swing calling conventions. UIManager can only be called on EDT, but ImageUtilities.loadImageIcon could be called from any thread so far.
I don't see anything related to EDT access restriction in UIManager's JavaDoc. Only in case of Nimbus l&f there are problems when retrieving values from UIManager - but that's because of poor implementation of that particular l&f.