[Vince]

The game defines a garage simply as a cube somewhere in the world. You will only experience a crash if you add a new garage door of the same model as the one that was removed. The game defines a specific object model to serve as the door to a garage and it will look for that object model in the vicinity when it attempts to open the garage. The crash is probably caused by the game trying to move said object but being unable to do so because the object in question was injected by SA-MP.