### 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);
	ERR_FAIL_COND_V(!is_inside_tree(), nullptr);
	Viewport *v = const_cast<Viewport *>(this);
	Window *w = Object::cast_to<Window>(v);
	while (!w) {
		v = v->get_parent_viewport();
		w = Object::cast_to<Window>(v);
	}
	return w;
}
void Viewport::_gui_remove_focus_for_window(Node *p_window) {