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
* @since 3.1
     */
    public ColorRegistry(Display display, boolean cleanOnDisplayDisposal) {
        Assert.isNotNull(display);
        this.display = display;
        if (cleanOnDisplayDisposal) {
			hookDisplayDispose();
		}
    }
    /**
     * Create a new <code>Color</code> on the receivers <code>Display</code>.
     * 
     * @param rgb the <code>RGB</code> data for the color.
     * @return the new <code>Color</code> object.
     * 
     * @since 3.1
     */
    private Color createColor(RGB rgb) {
        return new Color(display, rgb);
    }
    /**
     * Dispose of all of the <code>Color</code>s in this iterator.
     * 
     * @param iterator over <code>Collection</code> of <code>Color</code>
     */
    private void disposeColors(Iterator iterator) {
        while (iterator.hasNext()) {
            Object next = iterator.next();
            ((Color) next).dispose();
        }