cannot call method create_timer on a null value

Davalinchi · May 27, 2025, 4:27am

4.3.stable Godot Version Question

Godot seems to freak out when I call a timer every fourth scene.

to grossly oversimplify, every time when I get to the fourth scene and attempt to call the scene change to the fifth scene, Godot freaks out and does not let me do the scene change. It masks the issue as a timer issue.

I know that it is not a timer issue because of this:

both get_tree.await_timer("") and get_tree.change_scene_to_file("")

In GODOT issue 12,888 “What does get_tree() return?” the solution to it is

[1] get_tree() returns an (or better the) instance of this class. The instance itself then has means to access the nodes inside it. SceneTree itself is not a node, but a descendant of MainLoop. Think of it as a manager class that takes care of updating and managing all the nodes in it while running in an infinite loop until the player exits the game (get_tree().quit())

to grossly oversimplify, get_tree is basically

instantiating both the create_timer(n) and change_scene_to_file lines at once

or

calls await values first and then scene changes later to maximize performance and doing this in a timeframe so short that the difference between the beginning and end is so small that it might as well be instantaneous.

Why it does this at the fourth scene specifically I do not know, just because I typed this up like one of my college essays doesn’t mean I know what I am doing. I have only been doing this for nearly a year and a half, I know for a fact I do not know what I am doing.

To answer anyone’s questions in case they have any:

The script uses only a func _ready(): to handle all 2588 lines of code

the errors popped up at the part of the script that handles enemy AI and the part of the script that respawns the eight crystals needed to overpower enemy AI.

(this is the singleplayer part of a fighting game, so the AI is coded to move in a six point matrix within camera view, or to grossly simplify, something like this:)

the part of the script that freaked out was line 702 - line 705 this part of the script was one of the lines that handles enemy ai

702
703			await get_tree().create_timer(0.00900945946).timeout
704			$P2.position = Vector3(0,43.672,27.86767567567568)
705

where line 703 specifies the delay it takes for the position to be determined
and line 704 determines the position

this was calculated as a jump within 73 points on the line within 8.333 seconds

the other part of the script that freaked out was the part of the script that handles the crystal resizing or line 1359 to line 1384

1359		if $P1_Fight_GUI/Crystals/C1.size == Vector2(0,0) and $P1_Fight_GUI/Crystals/C2.size == Vector2(0,0) and $P1_Fight_GUI/Crystals/C3.size == Vector2(0,0) and $P1_Fight_GUI/Crystals/C4.size == Vector2(0,0) and $P1_Fight_GUI/Crystals/C5.size == Vector2(0,0) and $P1_Fight_GUI/Crystals/C6.size == Vector2(0,0) and $P1_Fight_GUI/Crystals/C7.size == Vector2(0,0) and $P1_Fight_GUI/Crystals/C8.size == Vector2(0,0):
1360	        await get_tree().create_timer(5).timeout
1361	        $P1_Fight_GUI/Crystals/C1.size = Vector2(59,61)
1362	        $P2_Fight_GUI/Crystals/C1.size = Vector2(59,61)
1363	        await get_tree().create_timer(5).timeout
1364		$P1_Fight_GUI/Crystals/C2.size = Vector2(40,45)
1365		$P2_Fight_GUI/Crystals/C2.size = Vector2(40,45)
1366		await get_tree().create_timer(5).timeout
1367		$P1_Fight_GUI/Crystals/C3.size = Vector2(40,45)
1368		$P2_Fight_GUI/Crystals/C3.size = Vector2(40,45)
1369		await get_tree().create_timer(5).timeout
1370		$P1_Fight_GUI/Crystals/C4.size = Vector2(40,45)
1371		$P2_Fight_GUI/Crystals/C4.size = Vector2(40,45)
1372		await get_tree().create_timer(5).timeout
1373		$P1_Fight_GUI/Crystals/C5.size = Vector2(40,45)
1374		$P2_Fight_GUI/Crystals/C5.size = Vector2(40,45)
1375		await get_tree().create_timer(5).timeout
1376		$P1_Fight_GUI/Crystals/C6.size = Vector2(40,45)
1377		$P2_Fight_GUI/Crystals/C6.size = Vector2(40,45)
1378		await get_tree().create_timer(5).timeout
1379		$P1_Fight_GUI/Crystals/C7.size = Vector2(40,45)
1380		$P2_Fight_GUI/Crystals/C7.size = Vector2(40,45)
1381		await get_tree().create_timer(5).timeout
1382		$P1_Fight_GUI/Crystals/C8.size = Vector2(40,45)
1383	        $P2_Fight_GUI/Crystals/C8.size = Vector2(40,45)
1384

to grossly oversimplify, the script handles this:

and because get_tree is calling create_timer(n) and change_scene_to_file("") simulataneously it means the game is broken for the player because the way GODOT was desgined was to stop the game if get_tree overlaps (for some reason)

the issue I have is that I don’t know how to program in a failsafe to prevent get_tree overlap
(so the player can access the other scene without the game crashing)

and because I don’t know how to code that in, it results in the game crashing at the fourth scene
(for reasons I do not know)

do you have methods to prevent such a crash from happening? if so, please comment them down below

[1] What does get_tree() return?

mrcdk · May 27, 2025, 4:41am

As explained in the SceneTree.change_scene_to_packed() documentation:

Note: Operations happen in the following order when change_scene_to_packed() is called:
The current scene node is immediately removed from the tree. From that point, Node.get_tree() called on the current (outgoing) scene will return null. current_scene will be null, too, because the new scene is not available yet.
At the end of the frame, the formerly current scene, already removed from the tree, will be deleted (freed from memory) and then the new scene will be instantiated and added to the tree. Node.get_tree() and current_scene will be back to working as usual.

So any subsequent call to get_tree() will return null. Make sure that when you are calling SceneTree.change_scene_to_file() you don’t access get_tree() until the scene has changed. Also make sure that you aren’t calling get_tree() in any place awaiting a timeout after you call change_scene_to_file().

You can guard the call like:

if get_tree():
    await get_tree().create_timer(5).timeout
    # do stuff

But, ideally, just use a Timer node if you are going to chain that many timers.

Davalinchi · May 27, 2025, 5:40am

this thread on github also goes over the scene change issue in case you need it for reference:

github.com/godotengine/godot

Errors when changing scene using change_scene_to_file()

opened 03:59PM - 06 Dec 23 UTC

golddotasksquestions

discussion topic:core

### Tested versions Reproducible in: 4.2 stable ### System information …Win 10, Godot 4.2 stable, Forward +, dedicated Nvidea RTX 1080 ### Issue description -------------------------------------------------- **Edit:** currently recommend workaround/fix seems to be to call `change_scene_to_file()` **deferred**. So instead of ```gdscript get_tree().change_scene_to_file("res://some_scene.tscn") ``` write: ```gdscript get_tree().change_scene_to_file.bind("res://some_scene.tscn").call_deferred() ``` -------------------------------------------------- -------------------------------------------------- Original issue description: Using `get_tree().change_scene_to_file("res://some_scene.tscn")` works , but it produces a number of red errors. These seem to be different depending on the circumstances. To test this, I made two scenes, **Scene A** and **Scene B** using Area3Ds and CharacterBody3D (with default template script): ![image](https://github.com/godotengine/godot/assets/47016402/72d41e41-ffa5-4d5f-8113-792abbaa0012) -------------------------------------------------- In **Scene A** I change scene by walking into an Area3D with a CharacterBody3D (template script), triggering a `body_entered` signal: ```gdscript extends Node3D func _on_area_3d_body_entered(body): if "Player" in body.name: get_tree().change_scene_to_file("res://change_3d_scene_b.tscn") ``` This will produce these two errors: > E 0:00:05:0401 change_3d_scene_A.gd:5 @ _on_area_3d_body_entered(): This function can't be used during the in/out signal. > <C++ Error> Condition "locked" is true. > <C++ Source> scene/3d/area_3d.cpp:289 @ _clear_monitoring() > <Stack Trace> change_3d_scene_A.gd:5 @ _on_area_3d_body_entered() > E 0:00:05:0401 change_3d_scene_A.gd:5 @ _on_area_3d_body_entered(): Removing a CollisionObject node during a physics callback is not allowed and will cause undesired behavior. Remove with call_deferred() instead. > <C++ Source> scene/3d/collision_object_3d.cpp:113 @ _notification() > <Stack Trace> change_3d_scene_A.gd:5 @ _on_area_3d_body_entered() -------------------------------------------------- In **Scene B** I change scene by walking into an Area3D with a CharacterBody3D, triggering a `body_entered` signal which will set a `can_interact` boolean variable in the character script. If the player then presses an Input action, the character fires a global custom signal, which the level connects to and changes the scene: Global Autoload: ```gdscript signal change_level ``` Character: ```gdscript var can_interact = false func _physics_process(delta): if can_interact: if Input.is_action_just_pressed("ui_accept"): Global.change_level.emit() ``` Level: ```gdscript extends Node3D func _ready(): Global.change_level.connect(change_to_other_level) func _on_area_3d_body_entered(body): if "Player" in body.name: body.can_interact = true func _on_area_3d_body_exited(body): if "Player" in body.name: body.can_interact = false func change_to_other_level(): get_tree().change_scene_to_file("res://change_3d_scene_a.tscn") ``` This will produce these four errors: > E 0:00:07:0043 change_3d_scene_b.tscn::GDScript_7cg2h:34 @ _physics_process(): Condition "!is_inside_tree()" is true. Returning: Transform3D() > <C++ Source> scene/3d/node_3d.cpp:343 @ get_global_transform() > <Stack Trace> change_3d_scene_b.tscn::GDScript_7cg2h:34 @ _physics_process() > E 0:00:07:0043 change_3d_scene_b.tscn::GDScript_7cg2h:34 @ _physics_process(): Condition "!is_inside_tree()" is true. Returning: Transform3D() > <C++ Source> scene/3d/node_3d.cpp:343 @ get_global_transform() > <Stack Trace> change_3d_scene_b.tscn::GDScript_7cg2h:34 @ _physics_process() > E 0:00:07:0043 change_3d_scene_b.tscn::GDScript_7cg2h:34 @ _physics_process(): Parameter "body->get_space()" is null. > <C++ Source> servers/physics_3d/godot_physics_server_3d.cpp:915 @ body_test_motion() > <Stack Trace> change_3d_scene_b.tscn::GDScript_7cg2h:34 @ _physics_process() > E 0:00:07:0043 change_3d_scene_b.tscn::GDScript_7cg2h:34 @ _physics_process(): Condition "!is_inside_tree()" is true. Returning: Transform3D() > <C++ Source> scene/3d/node_3d.cpp:343 @ get_global_transform() > <Stack Trace> change_3d_scene_b.tscn::GDScript_7cg2h:34 @ _physics_process() Note: Line 34 in the character script is just `move_and_slide()` -------------------------------------------------- Possibly related issue (however that's a different error): https://github.com/godotengine/godot/issues/84681 ### Steps to reproduce Use `get_tree().change_scene_to_file("res://some_scene.tscn")` as described above, or test the provided Minimal Reproduction Project below. ### Minimal reproduction project (MRP) [change_scene_errors.zip](https://github.com/godotengine/godot/files/13588297/change_scene_errors.zip)

Davalinchi · June 3, 2025, 3:10am

UPDATE: the problem was that the way I was calling scenes causes C# to crash

instead of:

get_tree().change_scene_to_file("res://your_file_location/your_scene.tscn")

its actually

get_tree().call_deferred("change_scene_to_file","res://your_file_location/your_scene.tscn")

yet again my strategy of copying solutions from forum boards saves the day again