This is observed with current 6u10 b07 pit build on all OSes.
Run SwingSet2 with Nimbus L&F set as a command line option. Open the JInternalFram demo and switch to Motif L&F.
Minimize a JInternalFrame and try to click on the JInternal Frame to bring up its popup menu.
The following exception occurs.

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Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
        at sun.font.FontDesignMetrics$MetricsKey.init(FontDesignMetrics.java:199)
        at sun.font.FontDesignMetrics.getMetrics(FontDesignMetrics.java:267)
        at sun.swing.SwingUtilities2.getFontMetrics(SwingUtilities2.java:954)
        at javax.swing.JComponent.getFontMetrics(JComponent.java:1597)
        at javax.swing.plaf.synth.SynthMenuItemUI.getPreferredMenuItemSize(SynthMenuItemUI.java:75)
        at javax.swing.plaf.synth.SynthMenuItemUI.getPreferredMenuItemSize(SynthMenuItemUI.java:618)
        at javax.swing.plaf.basic.BasicMenuItemUI.getPreferredSize(BasicMenuItemUI.java:360)
        at javax.swing.JComponent.getPreferredSize(JComponent.java:1632)
        at javax.swing.BoxLayout.checkRequests(BoxLayout.java:464)
        at javax.swing.BoxLayout.preferredLayoutSize(BoxLayout.java:281)
        at java.awt.Container.preferredSize(Container.java:1616)
        at java.awt.Container.getPreferredSize(Container.java:1601)
        at javax.swing.JComponent.getPreferredSize(JComponent.java:1634)
        at javax.swing.JPopupMenu.adjustPopupLocationToFitScreen(JPopupMenu.java:347)
        at javax.swing.JPopupMenu.getPopup(JPopupMenu.java:780)
        at javax.swing.JPopupMenu.setVisible(JPopupMenu.java:737)
        at javax.swing.JPopupMenu.show(JPopupMenu.java:910)
        at javax.swing.plaf.basic.BasicLookAndFeel$AWTEventHelper.eventDispatched(BasicLookAndFeel.java:2247)
        at java.awt.Toolkit$SelectiveAWTEventListener.eventDispatched(Toolkit.java:2353)
        at java.awt.Toolkit$ToolkitEventMulticaster.eventDispatched(Toolkit.java:2245)
        at java.awt.Toolkit.notifyAWTEventListeners(Toolkit.java:2203)
        at java.awt.Component.dispatchEventImpl(Component.java:4370)
        at java.awt.Container.dispatchEventImpl(Container.java:2116)
        at java.awt.Component.dispatchEvent(Component.java:4302)
        at com.sun.java.swing.plaf.motif.MotifDesktopIconUI$IconButton.forwardEventToParent(MotifDesktopIconUI.java:314)
        at com.sun.java.swing.plaf.motif.MotifDesktopIconUI$IconButton$2.mouseReleased(MotifDesktopIconUI.java:301)
        at java.awt.AWTEventMulticaster.mouseReleased(AWTEventMulticaster.java:272)
        at java.awt.AWTEventMulticaster.mouseReleased(AWTEventMulticaster.java:272)
        at java.awt.Component.processMouseEvent(Component.java:6100)
        at javax.swing.JComponent.processMouseEvent(JComponent.java:3265)
        at java.awt.Component.processEvent(Component.java:5865)
        at java.awt.Container.processEvent(Container.java:2058)
        at java.awt.Component.dispatchEventImpl(Component.java:4472)
        at java.awt.Container.dispatchEventImpl(Container.java:2116)
        at java.awt.Component.dispatchEvent(Component.java:4302)
        at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4322)
        at java.awt.LightweightDispatcher.processMouseEvent(Container.java:3986)

        at java.awt.LightweightDispatcher.dispatchEvent(Container.java:3916)
        at java.awt.Container.dispatchEventImpl(Container.java:2102)
        at java.awt.Window.dispatchEventImpl(Window.java:2440)
        at java.awt.Component.dispatchEvent(Component.java:4302)
        at java.awt.EventQueue.dispatchEvent(EventQueue.java:599)
        at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:273)
        at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:183)
        at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:173)
        at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:168)

        at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:160)

        at java.awt.EventDispatchThread.run(EventDispatchThread.java:121)

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