### 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)

if (path == p_path) {
		return;
	}
	path = p_path;
	vp_changed = true;
	if (vp) {
		vp->viewport_textures.erase(this);
		vp = nullptr;
	}
	if (proxy.is_valid() && proxy_ph.is_null()) {
		proxy_ph = RS::get_singleton()->texture_2d_placeholder_create();
		RS::get_singleton()->texture_proxy_update(proxy, proxy_ph);
	}
	if (get_local_scene() && !path.is_empty()) {
		setup_local_to_scene();
	} else {
		emit_changed();
	}
}
NodePath ViewportTexture::get_viewport_path_in_scene() const {