Groupcollide() not working in Alien Invasion project

I’m working on the Alien Invasion project from the 2nd Edition of Python Crash Course by Eric Matthes.
I am currently trying to make the alien and bullet disappear as they collide but when I add the following piece of code:
collitions = pygame.sprite.groupcollide(self.bullets, self.aliens, True, True)
The alien fleet quickly moves down to the bottom of the screen as soon as a bullet and alien collide.
You can find the full project code at: GitHub - pimbura/Alien-Invation at pimbura-patch-1-FleetGoesDownWhenShoot
Please help me if you can find the reason.