### Godot version

4.1.dev4

### System information

Windows 10 - Godot 4.1.dev4 - Compatibility

### Issue description

When you syncronize 2 variables and both are in watched mode (taking out the "sync" property too) the server crashes

### Steps to reproduce

Create a MultiplayerSynchronizer, export 2 variables from a script, connect the server and the client and the server will crash instantly

In the project I give, the server and client are connected automatically when you use 2 debug instances when pressing play

### Minimal reproduction project

[minimalproject.zip](https://github.com/godotengine/godot/files/11665652/minimalproject.zip)

MultiplayerSynchronizer *sync = get_id_as<MultiplayerSynchronizer>(p_sid);
	ERR_FAIL_COND(!sync); 
	Node *node = sync->get_root_node();
	ERR_FAIL_COND(!node); 
	const ObjectID oid = node->get_instance_id();
	if (spawned_nodes.has(oid) && p_peer != multiplayer->get_unique_id()) {
		_update_spawn_visibility(p_peer, oid);
	}
	_update_sync_visibility(p_peer, sync);
}
bool SceneReplicationInterface::is_rpc_visible(const ObjectID &p_oid, int p_peer) const {