Alien Invaders

Instructions:
left and right(arrow keys)
shoot(space)

Note:
First time making a greenfoot game. All you have to do is to shoot down the aliens as they descend. Avoid getting shot and make sure they don't touch the end as you will lose a life if that happens.
Bugs:
I set it for 3 lives, but you get an extra life at Lives: 0.
Game sometimes continues even if you lost all your lifes