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
inst = this;
	}
	/**
	 * Returns the image registry for this plugin.
	 *
	 * Where are the images?  The images (typically gifs) are found in the 
	 * same plugins directory.
	 *
	 * @see JFace's ImageRegistry
	 *
	 * Note: The workbench uses the standard JFace ImageRegistry to track its images. In addition 
	 * the class WorkbenchGraphicResources provides convenience access to the graphics resources 
	 * and fast field access for some of the commonly used graphical images.
	 */
	protected ImageRegistry createImageRegistry() {
		/* Just to be sure that we don't access this
		 * plug-ins image registry.
		 */
		Assert.isLegal(false);
		return WorkbenchImages.getImageRegistry();
	}
	
	public ImageRegistry getImageRegistry() {
		/* Just to be sure that we don't access this
		 * plug-ins image registry.
		 */
		Assert.isLegal(false);
		return null;
	}
	/* Return the default instance of the receiver. This represents the runtime plugin.