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
}
    /**
     * Returns the <code>color</code> associated with the given symbolic color 
     * name, or <code>null</code> if no such definition exists.
     * 
     * @param symbolicName symbolic color name
     * @return the <code>Color</code> or <code>null</code>
     */
    public Color get(String symbolicName) {
        Assert.isNotNull(symbolicName);
        Object result = stringToColor.get(symbolicName);
        if (result != null) {
			return (Color) result;
		}
        Color color = null;
        result = stringToRGB.get(symbolicName);
        if (result == null) {
			return null;
		}
        color = createColor((RGB) result);
        stringToColor.put(symbolicName, color);
        return color;
    }
    /* (non-Javadoc)
     * @see org.eclipse.jface.resource.ResourceRegistry#getKeySet()
     */
    public Set getKeySet() {
        return Collections.unmodifiableSet(stringToRGB.keySet());
    }
    /**