First of all, the layout is composed like this :
There are several istances of the sprite "Spawner" and a single istance of the three box sprites, designed to destroy at startup.
With this ^ ^ ^ code, the game crashes upon start. And yes, I need to have the for or the repeat before the pick, or else the engine won't choose a different "Spawner".
If this is a bug please fix as fast as possible, since it stopped the development of my game.