Looks up the node based on the unique node identifier.
        
        :param      nodeId

def findNodeById( self, objectId ):
        """
        Looks up the node based on the unique node identifier.
        
        :param      nodeId
        """
        for item in self.items():
            if ( isinstance(item, XNode) and item.objectId() == objectId):
                return item
        return None