Exactly! The theory is that wishing disaster on a fellow thespian removes the likleyhood of its occurance. Actors are notoriously superstitious - famous superstitions include not mentioning the title of 'Macbeth', but calling it 'The Scottish Play', and not whistling in your dressing room, which is bad luck. If you do whistle, you should deflect the bad luck by stepping outside the door, turning round three times, and then knocking before you re-enter.