Bug 140160 [Field Assist] - JAVADOC ONLY- FieldDecorationRegistry API should specify that null dec description is allowed
bundles/org.eclipse.jface/src/org/eclipse/jface/fieldassist/FieldDecorationRegistry.java
* 
	 * @param id
	 *            the String id of the decoration to be unregistered.
	 */
	public void unregisterFieldDecoration(String id) {
		decorations.put(id, null);
	}
	/**
	 * Returns the field decoration registered by the specified id .
	 * 
	 * @param id
	 *            the String id used to access the decoration.
	 * @return the FieldDecoration with the specified id, or <code>null</code>
	 *         if there is no decoration with the specified id.
	 */
	public FieldDecoration getFieldDecoration(String id) {
		Object entry = decorations.get(id);
		if (entry == null) {
			return null;
		}
		FieldDecoration dec = ((Entry) entry).getDecoration();
		Image image = dec.getImage();
		if (image != null) {
			maxDecorationHeight = Math.max(0, image.getBounds().height);
			maxDecorationWidth = Math.max(0, image.getBounds().width);
		}
		return dec;
	}
}