An already freed Object evaluates to true when used as a condition in if

[rakun99]

Godot version:
3.2.4 beta 4

OS/device including version:
Windows 7

Issue description:
Godot sends an error even when I make precaution that target doesn't Null.
I send a bunch of missiles with predefined target. First missile crashes the target and target is queue_freed. Because of that other missiles receive null, but I got an error.

Steps to reproduce:

Minimal reproduction project:

[Calinou]

@rakun99 Please upload a minimal reproduction project to make this easier to troubleshoot.

[KoBeWi]

You are supposed to use is_instance_valid() to avoid this error. So it should be either

if is_instance_valid(target):
    stuff

or

if not is_instance_valid(target):
    target = null

if target:
    stuff

That said, this error used to report [deleted object] instead of null. Not sure if this change is intended.

EDIT:
Related #41866
This is why if target doesn't work reliably in the first place. This is perfectly fine, but null and freed instance should be differentiated in the error message IMO.

EDIT2:
So actually it's already differentiated, but "null instance" sounds like it's null not a freed object.

EDIT 3:
Here's simple reproduction code:

func _ready() -> void:
	var node = Node.new()
	node.free()
	print(node.name)

The error:
Invalid get index 'name' (on base: 'null instance').

[Zireael07]

IIRC is_instance_valid() can be a fake positive too, so it isn't a fix...

[KoBeWi]

No, fake positives were fixed in the newest 3.2 releases.

[rakun99]

@rakun99 Please upload a minimal reproduction project to make this easier to troubleshoot.

Well, I used a steering behavior form KidsCanCode demo.

You are supposed to use is_instance_valid() to avoid this error. So it should be either

Wow, it worked. Thanks. So problem is solved.

[kleonc]

The same in v4.0.beta10.official [d0398f6]:

Related: #59816.

It's GDScript specific issue, right? 🤔

[KoBeWi]

@kleonc The screenshot you posted looks correct. The error reports the object as 'previously freed'.

[kleonc]

@kleonc The screenshot you posted looks correct. The error reports the object as 'previously freed'.

Oh, so the only issue in here is the error message saying "null instance" instead of something like "previously freed"?

[KoBeWi]

Well, the behavior is unintuitive, but the idea is that you have to use is_instance_valid() to make sure whether the object is freed or not. I think the fact that checking the object directly is inconsistent is due to performance reasons.

[vnen]

This is not a bug and from the discussion on #59816, it is expected behavior that if freed_object: evaluates to true. In general you should check why you are getting a freed object and if it is expected you should use is_instance_valid().