Get Blackboard Value with null input causes crash

Getting a blackboard value as actor and setting an actor to that on tick causes crash.

The real issue here is that you can use a function in BP (in this case `UBTFunctionLibrary::GetBlackboardValueAsObject`) with a hidden pin (in this case `NodeOwner` of type `UBTNode`) defaulting to "self" in a totally unrelated class' event graph (in this case `AIController`-derived BP class). And this blueprint compiles just fine without even a sign of warning.

1. Make a third person blueprint project
2. Create a new Blueprint > AIController
3. Add a tick event to the AI controller
4. Create an "Actor" type variable
5. From the Tick event, add a "Set" for the actor variable
6. Add a "Get Blackboard Value as Actor" node (may require turning off context sensitivity)
7. Promote the "Key" input to a variable.
8. Open the MyCharacter blueprint's default tab
9. Set the AIController class to the one created
10. PIE