How to get controller of mass actors in blueprint?

I am trying to get the AIController of an actor in the blueprint, so that I can call the Run Behavior Tree function and let the actor do something. If I drag the actor to the level, the Get Controller function simply works. But, if I use MassSpawner to generate the actors, the generated actors are not able to get the controller using the Get Controller function.

How can I get the controller of the mass actors?