Initializes the navigation system to start with the inputed root \
        item.
        
        :param      item | <XNavigationItem>

def setTopLevelItems( self, items ):
        """
        Initializes the navigation system to start with the inputed root \
        item.
        
        :param      item | <XNavigationItem>
        """
        if ( not self._navigationModel ):
            self.setNavigationModel(XNavigationModel(self))
        
        self._navigationModel.setTopLevelItems(items)