Program crushes..

[Sean11]

Hey guys i had open thread here and got much help from you last time so i encountered new problems
i created class called Bullet and when the bullet collide and enemy(spaceship) it is deleting the bullet,the spaceship, and spaceship pointer that is inside the vector
but when it hits the enemy the program crushes.. now i have tried to delete 1 spaceship from the spaceship row when the spaceships reach certain Y() but still when i remove from the scene 1 spaceship
even if i remove the spaceship from the scene, delete the certain spaceship from the vector and delete the object it self the program crushes. plz anyhelp
here is my code:
https://github.com/SeanR11/Spaceship---first-game
another question i got:
i want to add level(difficulty) class that incharge of changing levels from 1-30 on each level the spaceship ill be in different order or somthing like this any suggestions how to start it? what to inherits from attributes or methods?