Updates the callback command which is called when the ButtonGroup
        changes.

        Setting to `None` stops the callback.

        :param callback command:
            The callback function to call.

        :param callback args:
            A list of arguments to pass to the widgets `command`, defaults to
            `None`.

def update_command(self, command, args=None):
        """
        Updates the callback command which is called when the ButtonGroup
        changes.

        Setting to `None` stops the callback.

        :param callback command:
            The callback function to call.

        :param callback args:
            A list of arguments to pass to the widgets `command`, defaults to
            `None`.
        """
        if command is None:
            self._command = lambda: None
        else:
            if args is None:
                self._command = command
            else:
                self._command = utils.with_args(command, *args)