Calling custom function from other BP in behavior tree's task

So I was trying to follow this tutorial and in 20:07 there’s a cast to constructionProxyBP from which we try to get it’s function ‘Report To Shift’ and when I try to do that I always get an “Acces none trying to read property…” error. Can someone explain to my how can I call this kind of fuction in behavior tree’s task?