You could initialize your blackboard value before you run your behavior tree, or alternatively you could wait until your blackboard value is set before calling run.
https://docs.unrealengine.com/latest/INT/Engine/AI/BehaviorTrees/QuickStart/6/index.html
A null pointer or uninitialized value can be annoying, I definitely agree that it shouldn’t crash script, but it also helps point to a design pattern consideration. One of two methods I mentioned might be more graceful for your implementation than a benign wait.