### Godot version

v4.0 rc6

### System information

Windows 10

### Issue description

When using a ViewportTexture from a SubViewport on a TextureRect, it throws errors after running the scene once and then switching scenes in the editor.
![Pasted image 20230227212626](https://user-images.githubusercontent.com/44676282/221680047-984fa03e-33d1-4856-9a7f-1700b28423cf.png)


### Steps to reproduce

Run the canvas-scene once.
Switch to another scene in the editor.
Switch back to the canvas-scene

### Minimal reproduction project

[ViewportTextureError.zip](https://github.com/godotengine/godot/files/10844099/ViewportTextureError.zip)

ERR_READ_THREAD_GUARD_V(false);
	return override_canvas_transform;
}
void Viewport::set_canvas_transform_override(const Transform2D &p_transform) {
	ERR_MAIN_THREAD_GUARD;
	if (canvas_transform_override == p_transform) {
		return;
	}
	canvas_transform_override = p_transform;
	if (override_canvas_transform) {
		RenderingServer::get_singleton()->viewport_set_canvas_transform(viewport, find_world_2d()->get_canvas(), canvas_transform_override);
	}
}
Transform2D Viewport::get_canvas_transform_override() const {
	ERR_READ_THREAD_GUARD_V(Transform2D());
	return canvas_transform_override;
}
void Viewport::set_canvas_transform(const Transform2D &p_transform) {
	ERR_MAIN_THREAD_GUARD;
	canvas_transform = p_transform;
	if (!override_canvas_transform) {
		RenderingServer::get_singleton()->viewport_set_canvas_transform(viewport, find_world_2d()->get_canvas(), canvas_transform);
	}
}
Transform2D Viewport::get_canvas_transform() const {