### 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 (int i = 0; i < gui.sub_windows.size(); i++) {
		if (gui.sub_windows[i].window == p_window) {
			return i;
		}
	}
	return -1;
}
void Viewport::_update_viewport_path() {
	if (viewport_textures.is_empty()) {
		return;
	}
	Node *scene_root = get_scene_file_path().is_empty() ? get_owner() : this;
	if (!scene_root && is_inside_tree()) {
		scene_root = get_tree()->get_edited_scene_root();
	}
	if (scene_root && (scene_root == this || scene_root->is_ancestor_of(this))) {
		NodePath path_in_scene = scene_root->get_path_to(this);
		for (ViewportTexture *E : viewport_textures) {
			E->path = path_in_scene;
		}
	}
}
void Viewport::_notification(int p_what) {