Looks up the node based on the inputed node name.
        
        :param      objectName     | <str>

def findNode( self, objectName ):
        """
        Looks up the node based on the inputed node name.
        
        :param      objectName     | <str>
        """
        for item in self.items():
            if ( isinstance(item, XNode) and 
                 item.objectName() == objectName ):
                return item
        return None