    def _on_selection(self):
        """
        Internal function to handle directory traversal or bubble notifications up to user of the
        Widget as needed.
        """
        if self.value and os.path.isdir(self.value):
            self._populate_list(self.value)
        elif self._external_notification:
            self._external_notification()
Internal function to handle directory traversal or bubble notifications up to user of the
Widget as needed.
asciimatics/widgets/filebrowser.py