This is observed on current 6u10 b07 pit build on all OSes when switching to Nimbus L&F at runtime.
Before the fix 6595331 in this PIT, switching to any other L&F from Nimbus had some issues(6594220).
The Fix in 6595331 which addresses
"Misc: - Added clearing of the image cache when the LAF is uninstalled"

has now caused a regression.
I can switch from Nimbus to other L&Fs and the new L&Fs are applied correctly. (The bug 6594220 should be closed as fixed)
However when I load an application in Metal/Windows/etc. L&F and then switch to Nimbus it throws an NPE as below:
----------START STACK TRACE--------------

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
        at javax.swing.plaf.synth.SynthComboBoxUI$SynthComboBoxRenderer.getListCellRendererComponent(SynthComboBoxUI.java:377)
        at sun.swing.plaf.synth.SynthFileChooserUIImpl$DirectoryComboBoxRenderer.getListCellRendererComponent(SynthFileChooserUIImpl.java:651)
        at javax.swing.plaf.basic.BasicListUI.updateLayoutState(BasicListUI.java:1342)
        at javax.swing.plaf.basic.BasicListUI.maybeUpdateLayoutState(BasicListUI.java:1292)
        at javax.swing.plaf.basic.BasicListUI$Handler.valueChanged(BasicListUI.java:2605)
        at javax.swing.DefaultListSelectionModel.fireValueChanged(DefaultListSelectionModel.java:167)
        at javax.swing.DefaultListSelectionModel.fireValueChanged(DefaultListSelectionModel.java:147)
        at javax.swing.DefaultListSelectionModel.fireValueChanged(DefaultListSelectionModel.java:194)
        at javax.swing.DefaultListSelectionModel.changeSelection(DefaultListSelectionModel.java:388)
        at javax.swing.DefaultListSelectionModel.changeSelection(DefaultListSelectionModel.java:398)
        at javax.swing.DefaultListSelectionModel.setSelectionInterval(DefaultListSelectionModel.java:442)
        at javax.swing.JList.setSelectedIndex(JList.java:2179)
        at javax.swing.plaf.synth.SynthComboPopup.configureList(SynthComboPopup.java:47)
        at javax.swing.plaf.basic.BasicComboPopup.<init>(BasicComboPopup.java:315)
        at javax.swing.plaf.synth.SynthComboPopup.<init>(SynthComboPopup.java:26)
        at javax.swing.plaf.synth.SynthComboBoxUI.createPopup(SynthComboBoxUI.java:192)
        at javax.swing.plaf.basic.BasicComboBoxUI.installUI(BasicComboBoxUI.java:223)
        at javax.swing.plaf.synth.SynthComboBoxUI.installUI(SynthComboBoxUI.java:83)
        at javax.swing.JComponent.setUI(JComponent.java:673)
        at javax.swing.JComboBox.setUI(JComboBox.java:238)
        at javax.swing.JComboBox.updateUI(JComboBox.java:247)
        at javax.swing.SwingUtilities.updateComponentTreeUI0(SwingUtilities.java:1206)
        at javax.swing.SwingUtilities.updateComponentTreeUI0(SwingUtilities.java:1221)
        at javax.swing.SwingUtilities.updateComponentTreeUI0(SwingUtilities.java:1221)
        at javax.swing.SwingUtilities.updateComponentTreeUI(SwingUtilities.java:1197)
        at FileChooserDemo$OptionListener.actionPerformed(FileChooserDemo.java:657)
        at javax.swing.JComboBox.fireActionEvent(JComboBox.java:1242)
        at javax.swing.JComboBox.setSelectedItem(JComboBox.java:569)
        at javax.swing.JComboBox.setSelectedIndex(JComboBox.java:605)
        at javax.swing.plaf.basic.BasicComboPopup$Handler.mouseReleased(BasicComboPopup.java:816)
        at java.awt.AWTEventMulticaster.mouseReleased(AWTEventMulticaster.java:273)
        at java.awt.Component.processMouseEvent(Component.java:6100)
        at javax.swing.JComponent.processMouseEvent(JComponent.java:3265)
        at javax.swing.plaf.basic.BasicComboPopup$1.processMouseEvent(BasicComboPopup.java: 480)
        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)

-----------END STACK TRACE---------------