_compute_axis indexed config.motors[motor_id] directly without
checking the array length. For motor_id=4 (the synthetic external-
axis motor used by ExternalAxis) there is no entry in config.motors,
so motor was undefined and motor["homing-mode"] threw - which
made the entire 'a' computed prop return undefined and the A row
never rendered.
Default to {} when the index is out of bounds.
Henrik Muehe
7cdab010b3