Why does this code give me the error message "Attempt to call function 'change_scene' in base 'null instance'."?

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:bust_in_silhouette: Asked By Cashew 
var error_code: int = get_tree().change_scene("res://Levels/Level1.tscn")

Probably this?

https://forum.godotengine.org/97726/why-null-instance-on-get_tree#:~:text=1%20Answer&text=get_tree()%20will%20return%20null,()%20wherever%20it%20needs%20to.

jgodfrey | 2023-02-01 17:56

The thing is I want to assign get_tree().change_scene("res://Levels/Level1.tscn") as an int variable so I can print is if it is not equal to 0
The reason for this stems from the Warning message where get_tree().change_scene("res://Levels/Level1.tscn") tells you it returns a value but it is never used.

Cashew | 2023-02-01 18:00

I’m not sure I follow exactly, but here’s some background that might be helpful.

First, get_tree() is a method associated with anything inheriting from Node. However, it will return null if the node from which it’s called IS NOT a part of the scene tree (that’s outline in the link I provided above).

So, assuming you call it from a node that’s not in the scene tree, you cannot then immediately try to call change_scene() because you’ll be working with a null reference.

So, if there’s a chance you might be calling get_tree() from a node not in the scene tree, do that first. That way you can verify that you have a valid reference before tyring to use it. So (untested) but something like this:

var tree = get_tree()
if tree:
    var error_code: int = tree.change_scene("res://Levels/Level1.tscn")
    if error_code != OK:
        print("failed to load requested scene")
else:
    print("failed to get tree")

jgodfrey | 2023-02-01 19:24

Thank you very much, I am quite new to Godot and coding so your info is much appreciated!

Cashew | 2023-02-02 21:05