I tried everything as per the lecture, yet the editor crashes on a None error, which would often show up as a None error accessing the Controller without crashing the editor on the message log.
Update: the editor crashes if I kill even a single bot, but is otherwise safe. Switched the controller to the Old Controller reference even in the Kill Character/Player method. Does Event Unpossessed help in any way?
What are the steps to crash? Doesn’t crash if I kill an enemy unless I misunderstood what crashes.
I fixed it. 
Basically detach controllers for AI and commit to the Un Possess method for player controller only, using a simple branch.