### 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)

for (const ObjectID &id : gui.canvas_parents_with_dirty_order) {
		Object *obj = ObjectDB::get_instance(id);
		if (!obj) {
			continue; 
		}
		Node *n = static_cast<Node *>(obj);
		for (int i = 0; i < n->get_child_count(); i++) {
			Node *c = n->get_child(i);
			CanvasItem *ci = Object::cast_to<CanvasItem>(c);
			if (ci) {
				ci->update_draw_order();
				continue;
			}
			CanvasLayer *cl = Object::cast_to<CanvasLayer>(c);
			if (cl) {
				cl->update_draw_order();
			}
		}
	}
	gui.canvas_parents_with_dirty_order.clear();
}
void Viewport::_sub_window_update_order() {