Bug 4864304 made various performance enhancements to Swing. One of the things it did was to allow null for the horizontal scroll bar of a JScrollPane. It then changed BasicComboPopup to set the horizontal scrollbar of the scrollpane to null.
After this fix, when using a JComboBox to switch the look and feel from Metal, a NullPointerException is thrown from MetalScrollPaneUI.uninstallUI (which doesn't check for null). A test case is attached (JSliderTest.java). Run it, and switch to another L&F.
Note: I have another test case which does something similar (ListTest.java), but it isn't affected. I don't know why - could this be timing related?