docs: def checkablePopup( self ):
          """
          Returns the popup if this widget is checkable.
          
          :return     <QListView> || None
          """
          if not self._checkablePopup and self.isCheckable():
              popup = QListView(self)
              popup.setSelectionMode(QListView.NoSelection)
              popup.setHorizontalScrollBarPolicy(Qt.ScrollBarAlwaysOff)
              popup.setWindowFlags(Qt.Popup)
              popup.installEventFilter(self)
              popup.doubleClicked.connect(self.checkModelIndex)
              self._checkablePopup = popup
          
          return self._checkablePopup