Hi,
what you could do is save the said actor instance as a blackboard variable in the controller upon begin play, for instance. There is a blueprint node, something like “set blackboard value as actor” (or something similar). You need to hook in the function “get blackboard”.
Of course this will only work, if you set up the behaviour tree / blackboard in the controller beforehand (also on begin play).
Then you can access the actor from the blackboard in the behaviour tree.
I hope this helps.
Best!