I wouldn’t recommend accessing the AI Behavior Tree from the Animation Blueprint, it seems somewhat hacky to me and not Object Oriented. If you want your AnimBP to know something about your Behavior Tree then expose this data through your AI Pawn/Character blueprint which is the one that usually communicates with the AnimBP. For example you could create an enumeration for state variable on your AI Pawn/Character blueprint that gets set only inside the Behavior Tree and you would access and read this variable inside your AnimBP.
This way your AnimBP does not know anything about your Behavior Tree and your Behavior Tree does not know anything about your AnimBP since your AI Pawn/Character blueprint is acting as the mediator.