Sphere Trace Causes This Error: Accessed None 'CallFunc_BreakhitResult_HitActor2'

I’ve been trying to get a simple detection system working that allows an AI controlled pawn to follow the player, exactly as its described in the Behavior Tree Tutorial. However when I updated to 4.7 everything broke, and I replaced all the nodes that needed to be replaced. Except that now its giving me this result after running in the editor (and the AI pawn running to a angled location before returning to its origin point as a blackboard key is invalid)


Everything is set up as it is in the Behavior Tree Tutorial.
I’m stuck trying to figure out why this sphere trace is causing this error, as it is stopping the blueprint from assigning the player’s pawn to the TargetToFollow blackboard key, which stops the BT right from the start. Anyone know why its doing this?
Using Unreal 4.7.3
Thanks for any help!