def model(self):
        """ Returns the ConfigTreeModel this item belongs to.
            If the model is None (not set), it will use and cache the parent's model.
            Therefore make sure that an ancestor node has a reference to the model! Typically by
            setting the model property of the invisible root item in the model constructor.
        """
        if self._model is None and self.parentItem is not None:
            self._model = self.parentItem.model
        return self._model