Returns the navigator associated with this box.
        
        :return     <XOrbColumnNavigator>

def navigator(self):
        """
        Returns the navigator associated with this box.
        
        :return     <XOrbColumnNavigator>
        """
        if self._navigator is None:
            self._navigator = nav = XOrbColumnNavigator()
            self.lineEdit().setCompleter(None)
            
            # initialize settings
            nav.hide()
            nav.header().hide()
            nav.setWindowFlags(Qt.Popup)
            nav.setFocusPolicy(Qt.ClickFocus)
            nav.installEventFilter(self)
            nav.setVerticalScrollMode(nav.ScrollPerPixel)
            
            # create connections
            nav.itemClicked.connect(self.acceptColumn)
            self.lineEdit().textEdited.connect(self.showPopup)
            nav.schemaColumnChanged.connect(self.emitSchemaColumnChanged)
        
        return self._navigator