def model_changed(self, model):
        """Apply the model to the combobox

        When a level instance is created, the model is None. So it has to be set afterwards.
        Then this method will be called and your level should somehow use the model

        :param model: the model that the level should use
        :type model: QtCore.QAbstractItemModel
        :returns: None
        :rtype: None
        :raises: None
        """
        self.setModel(model)
        # to update all lists belwo
        # current changed is not triggered by setModel somehow
        if model is not None:
            self.setCurrentIndex(self.model().index(0, 0))
Apply the model to the combobox

        When a level instance is created, the model is None. So it has to be set afterwards.
        Then this method will be called and your level should somehow use the model

        :param model: the model that the level should use
        :type model: QtCore.QAbstractItemModel
        :returns: None
        :rtype: None
        :raises: None