### 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(nullptr);
	return camera_2d;
}
Transform2D Viewport::get_final_transform() const {
	ERR_READ_THREAD_GUARD_V(Transform2D());
	return stretch_transform * global_canvas_transform;
}
void Viewport::assign_next_enabled_camera_2d(const StringName &p_camera_group) {
	ERR_MAIN_THREAD_GUARD;
	List<Node *> camera_list;
	get_tree()->get_nodes_in_group(p_camera_group, &camera_list);
	Camera2D *new_camera = nullptr;
	for (Node *E : camera_list) {
		Camera2D *cam = Object::cast_to<Camera2D>(E);
		if (!cam) {
			continue; 
		}
		if (cam->is_enabled()) {
			new_camera = cam;
			break;
		}
	}
	_camera_2d_set(new_camera);
	if (!camera_2d) {
		set_canvas_transform(Transform2D());
	}
}
void Viewport::_update_canvas_items(Node *p_node) {