Fixed errors being caused by using NULL entities, and put in some :IsValid()s where they should've been

The server errors were being caused by this:

Constraints breaking are detected by an EntityRemoved hook. Whenever it detects an entity that is a constraint being removed, the script waits for 0.05 seconds. After that, it checks if both of the constraint's parents are nonexistent. This is to stop jumpscaring players with sounds when an entire contraption is removed at once (e.g. undoing a dupe). The problem is that it only checked if BOTH parents didn't exist, so if one parent still existed but the other one didn't, it would try to check the material of the nonexistent parent and throw an error. This update fixes it by making the script check if at least ONE parent exists.

The client errors were solved by putting in some :IsValid()s where they should've been.

Added BANG effects for when props of the appropriate material hit something hard