Stream of NullPointerException is being thrown when user hovers mouse over the combo box if Look and Feel is changed to Windows Classic. Run attached application on Win XP and change laf to all available lafs in the combo. When changing to Windows Classic exceptions will be thrown. Tested on Mustang b92 build.

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
        at com.sun.java.swing.plaf.windows.WindowsComboBoxUI$RolloverListener.mouseEntered(WindowsComboBoxUI.java:287)
        at java.awt.AWTEventMulticaster.mouseEntered(AWTEventMulticaster.java:241)
        at java.awt.Component.processMouseEvent(Component.java:6044)
        at javax.swing.JComponent.processMouseEvent(JComponent.java:3240)
        at java.awt.Component.processEvent(Component.java:5800)
        at java.awt.Container.processEvent(Container.java:2045)
        at java.awt.Component.dispatchEventImpl(Component.java:4407)
        at java.awt.Container.dispatchEventImpl(Container.java:2103)
        at java.awt.Component.dispatchEvent(Component.java:4237)
        at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4309)
        at java.awt.LightweightDispatcher.trackMouseEnterExit(Container.java:4098)
        at java.awt.LightweightDispatcher.processMouseEvent(Container.java:3955)
        at java.awt.LightweightDispatcher.dispatchEvent(Container.java:3903)
        at java.awt.Container.dispatchEventImpl(Container.java:2089)
        at java.awt.Window.dispatchEventImpl(Window.java:2427)
        at java.awt.Component.dispatchEvent(Component.java:4237)
        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)