Comment 10 for bug 1770901

> I think the better solution would be to check
> if the immovable is NIL before trying to remove it.

I have to repeat that checking for sentry's existence doesn't solve the scenario issue. When the sentry vanishes, there is no more option to conquer it. Furthermore, upon gathering of the wares, the monastery got self-destroyed, not giving the reward. Maybe the scenario code assumed that the sentry got conquered by the player.