Ahh right, thanks for pointing that out! Turns out that at some point I had connected mouse_entered from the Tree StaticBody2D as well, which is why the error was shown.
Running the Tree scene now everything seems to work, when I try chopping the trees in my MainFarm scene though, it doesn't seem to register my mouse anymore. Do I need to tell my MainFarm Node2D what to do with mouse input separately? Can the Nodes on top of my Trees maybe block mouse detection?

EDIT: Turning off CanvasLayer fixes mouse detection, so I'm working on it right now, no need to answer my questions above!