Whenever I see "(on base:'null instance')" it usually means I tried to reference a node but got its location in the scene tree wrong. This happens easily if your enemy node's script was tested using a relative path to the player node, but is then instanced somewhere else in the scene tree that breaks that path.
Assuming it is, there are a few things to consider/get around it including but not limited to:
ensuring relative paths account for instancing location (what I usually do if not using groups or detecting post instancing with some FoV logic for example)
creating/adding nodes to a group querying said group (possibly a little slower, but useful for other reasons. Often will use this anyway)
setting an absolute path (Moves the problem to if you ever move the player node, but at least now the enemy node can be instanced anywhere in the scene tree and is going to be as performant as relative paths)
outright using find_node from root (the blunt force "it works" approach, though fairly sure this is going to be the slowest method, albeit slight)
Probably other ways too, but this is usually how I go about it.
This should help a bit regarding relative/absolute paths:
https://kidscancode.org/godot_recipes/basics/getting_nodes/
If it's none of the above, can you share the relevant code snippets/screenshot of your scene tree (Remote & Local while playing) as this should help.
