Bug 189636 Custom rule template failing
Null poonter exception being thrown when trying to add a custom rule which is causing the rule cretion to fail. Also there is a minor typo in the rule template string.
bundles/org.eclipse.ui.navigator/src/org/eclipse/ui/internal/navigator/NavigatorContentService.java
* <b>possibleChildren</b> expression.
	 * 
	 * @param anElement
	 *            The element to use in the query
	 * @param toComputeOverrides
	 * 			 True indicates the overridden tree should be traversed.
	 * @return The set of {@link INavigatorContentDescriptor}s that are
	 *         <i>visible</i> and <i>active</i> for this content service and
	 *         have a <b>possibleChildren</b> expression that is <i>enabled</i>
	 *         for the given element.
	 */
	public Set findDescriptorsWithPossibleChild(Object anElement, boolean toComputeOverrides) {
		NavigatorContentDescriptor descriptor = getSourceOfContribution(anElement);
		Set result = new TreeSet(ExtensionPriorityComparator.INSTANCE);
		if (descriptor != null) {
			result.add(descriptor);
		}
		result.addAll(CONTENT_DESCRIPTOR_REGISTRY
				.findDescriptorsForPossibleChild(anElement, assistant, toComputeOverrides));
		return result;
	}	
	/*
	 * (non-Javadoc)
	 * 
	 * @see org.eclipse.ui.internal.navigator.INavigatorContentService#onExtensionActivation(java.lang.String,
	 *      java.lang.String, boolean)
	 */
	public void onExtensionActivation(String aViewerId,
			String[] aNavigatorExtensionId, boolean toEnable) {
		synchronized (this) {