### 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)

// SubViewports, that are used as Textures are not considered to be directly attached to screen.
	return Object::cast_to<SubViewportContainer>(get_parent()) && get_parent()->get_viewport() && get_parent()->get_viewport()->is_directly_attached_to_screen();
}
bool SubViewport::is_attached_in_viewport() const {
	return Object::cast_to<SubViewportContainer>(get_parent());
}
void SubViewport::_notification(int p_what) {
	ERR_MAIN_THREAD_GUARD;
	switch (p_what) {
		case NOTIFICATION_ENTER_TREE: {
			RS::get_singleton()->viewport_set_active(get_viewport_rid(), true);
			SubViewportContainer *parent_svc = Object::cast_to<SubViewportContainer>(get_parent());
			if (parent_svc) {
				parent_svc->recalc_force_viewport_sizes();
			}
		} break;
		case NOTIFICATION_EXIT_TREE: {
			RS::get_singleton()->viewport_set_active(get_viewport_rid(), false);
		} break;
	}
}
void SubViewport::_bind_methods() {