In all projects (Indie and Pro) at the Arrow script, the line:
onready var head: Line2D = $Head
should be replaced by:
onready var head: Sprite = $Head
Godot didn't detect that until 3.2.2.
Thanks! A pity the type checker didn't pick that up before. It's because for a long time in the project, the head of the arrow was actually a Line2D node.
I'll add an errata note on the lesson where this is first introduced.
Ah I saw this only happens in the Godot projects and the code shown in the video appears to be correct. I'll fix that right away!
The same error happens in the CameraAnchorDetector script.