Bug 51110 [Preferences] Error notifying a preference listener on font change.
M6 Create a font definition with no defaultsTo attribute. Go in to preferences and change the value of Text Font and hit OK you will get an error because you try call JFaceResources.getFontRegistry().put() with a null value.
bundles/org.eclipse.ui/src/org/eclipse/ui/internal/UIPlugin.java
/**
 * This class represents the TOP of the workbench UI world
 * A plugin class is effectively an application wrapper
 * for a plugin & its classes. This class should be thought
 * of as the workbench UI's application class.
 *
 * This class is responsible for tracking various registries
 * font, preference, graphics, dialog store.
 *
 * This class is explicitly referenced by the 
 * workbench plugin's  "plugin.xml" and places it
 * into the UI start extension point of the main
 * overall application harness
 *
 * When is this class started?
 *      When the Application
 *      calls createExecutableExtension to create an executable
 *      instance of our workbench class.
 */
public class UIPlugin extends AbstractUIPlugin {
	
	private static UIPlugin inst;
	
	/**
	 * Create an instance of the WorkbenchPlugin.
	 * The workbench plugin is effectively the "application" for the workbench UI.
	 * The entire UI operates as a good plugin citizen.
	 */
	public UIPlugin(IPluginDescriptor descriptor) {
		super(descriptor);