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
/**
     * This registries <code>Display</code>. All colors will be allocated using 
     * it.
     */
    protected Display display;
    /**
     * Collection of <code>Color</code> that are now stale to be disposed when 
     * it is safe to do so (i.e. on shutdown).
     */
    private List staleColors = new ArrayList();
    /**
     * Table of known colors, keyed by symbolic color name (key type: <code>String</code>,
     * value type: <code>org.eclipse.swt.graphics.Color</code>.
     */
    private Map stringToColor = new HashMap(7);
    /**
     * Table of known color data, keyed by symbolic color name (key type:
     * <code>String</code>, value type: <code>org.eclipse.swt.graphics.RGB</code>).
     */
    private Map stringToRGB = new HashMap(7);
    /**
     * Runnable that cleans up the manager on disposal of the display.