Bug 53881 [Colors] Color/Gradient registry should return default values.
There should be default values for when a user does .get() on a bad key.
bundles/org.eclipse.jface/src/org/eclipse/jface/resource/GradientRegistry.java
Gradient gradient = createGradient(symbolicName, (GradientData) result);
		if (gradient != null)
			stringToGradient.put(symbolicName, gradient);
		return gradient;
	}
	
	/* (non-Javadoc)
	 * @see org.eclipse.jface.resource.ResourceRegistry#getKeySet()
	 */
	public Set getKeySet() {	    
	    return Collections.unmodifiableSet(stringToGradientData.keySet());
	}
	/**
	 * Returns the gradient data associated with the given symbolic gradient name.
	 *
	 * @param symbolicName symbolic gradient name.
	 * @return the <code>GradientData</code> data.
	 */
	public GradientData getGradientData(String symbolicName) {
		Assert.isNotNull(symbolicName);
		return (GradientData) stringToGradientData.get(symbolicName);
	}
	/* (non-Javadoc)
	 * @see org.eclipse.jface.resource.ResourceRegistry#clearCaches()
	 */
	protected void clearCaches() {
		disposeGradients(stringToGradient.values().iterator());
		disposeGradients(staleGradients.iterator());