Bug 182761 [Themes] Undefined color entry will cause an NPE if looked up
If you look up a ColorDescriptor from a theme and that color is not registered you will get a descriptor with a null color that will result in an NPE. We should either get back null, handle the null descriptor or have some API we can check.
bundles/org.eclipse.jface/src/org/eclipse/jface/resource/ColorRegistry.java
*/
    protected Runnable displayRunnable = new Runnable() {
        public void run() {
            clearCaches();
        }
    };
    /**
     * Create a new instance of the receiver that is hooked to the current 
     * display.
     * 
     * @see org.eclipse.swt.widgets.Display#getCurrent()
     */
    public ColorRegistry() {
        this(Display.getCurrent(), true);
    }
    /**
     * Create a new instance of the receiver.
     * 
     * @param display the <code>Display</code> to hook into.
     */
    public ColorRegistry(Display display) {
    	this (display, true);
    }
    /**
     * Create a new instance of the receiver.
     * 
     * @param display the <code>Display</code> to hook into
     * @param cleanOnDisplayDisposal
	 *            whether all fonts allocated by this <code>ColorRegistry</code>
	 *            should be disposed when the display is disposed