Hello, while following the lesson I am getting the above error. I was following the lesson and have even copy/pasted the code at this point and still am getting this error. Simulation and EntityTracker code I have also now copy/pasted so I am not sure what is going on.
this suggests that the _tracker variable hasn't been initialized. The thing is you should get a null error, but you assigned the EntityTracker class to the _track variable. While in the series we don't assign the class, we just set the type hint. Notice how, unlike in your code, we don't use the equal sign.
var _tracker: EntityTracker
Then, this EntityTracker variable needs a reference to an instance of the EntityTracker class. We provide it from the Simulation.gd script:
var _tracker := EntityTracker.new()
func _ready() -> void:
_entity_placer.setup(_tracker, _ground, _player)
We have to call this setup() function to provide the entity tracker to our node, otherwise, we can't keep track of entities on the game board.
I hope this helps.