### Godot version

4.2 eca6f0e

### System information

Windows 10 x64

### Issue description


https://github.com/godotengine/godot/assets/2223172/a2796263-4e2e-4b14-a080-b3ad034cc141



### Steps to reproduce

1. Add SubViewport
2. Add Sprite2D
3. Set spirte's texture to ViewportTexture
4. Create new scene
5. Instance the new scene inside the scene with viewport
6. Make sure both scenes are opened
7. Go to instance scene and save it
8. Go to viewport scene

You will see error spam. Reloading the scene fixes it.

### Minimal reproduction project

[vip.zip](https://github.com/godotengine/godot/files/12292386/vip.zip)

ERR_READ_THREAD_GUARD_V(false);
	return size_2d_override_stretch;
}
void SubViewport::set_update_mode(UpdateMode p_mode) {
	ERR_MAIN_THREAD_GUARD;
	update_mode = p_mode;
	RS::get_singleton()->viewport_set_update_mode(get_viewport_rid(), RS::ViewportUpdateMode(p_mode));
}
SubViewport::UpdateMode SubViewport::get_update_mode() const {
	ERR_READ_THREAD_GUARD_V(UPDATE_DISABLED);
	return update_mode;
}
void SubViewport::set_clear_mode(ClearMode p_mode) {
	ERR_MAIN_THREAD_GUARD;
	clear_mode = p_mode;
	RS::get_singleton()->viewport_set_clear_mode(get_viewport_rid(), RS::ViewportClearMode(p_mode));
}
SubViewport::ClearMode SubViewport::get_clear_mode() const {
	ERR_READ_THREAD_GUARD_V(CLEAR_MODE_ALWAYS);
	return clear_mode;
}
DisplayServer::WindowID SubViewport::get_window_id() const {
	ERR_READ_THREAD_GUARD_V(DisplayServer::INVALID_WINDOW_ID);
	return DisplayServer::INVALID_WINDOW_ID;
}
Transform2D SubViewport::get_screen_transform_internal(bool p_absolute_position) const {