Returns the popup if this widget is checkable.
        
        :return     <QListView> || None

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