on your begin overlap event script, right click the as tree pin on the cast and select promote to variable. this variable that is created will be a reference to the tree your overlapping. so from there all you will need to do is plug that variable (a get of the variable) into the object pin. actually once you have the variable setup you shouldnt actually need the cast where your getting the variables.
below is a picture of how you could modify your script to get it to work.
honestly i dont think this is the most optimal way to implement this functionality, personally i would have all of this scripting in the tree bp, but thats just my opinion.
