Creating a variable does not do anything on its own, you need to assign it value, too. When you spawn something you can:
But, when using dispatchers the way you do, there is no need for a hard reference at all, I would actually get rid of it and just:
Now you can just destroy the tree (without even calling a dispatcher) and the spawner will receive the custom event call with the reference of the tree that was destroyed.
You also do not need an additional custom dispatcher. Actors already have a bunch and this onDestroyed will work well here.
Bind the OnDestroyed here:
Use Assign to automagically create a custom event with the correct signature (pins with types). The best part here is, that you do not need to store ActorBP_Tree reference anywhere. They all bind to the same custom event and each will execute individually.