### 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)

Node *node = Object::cast_to<Node>(p_obj);
	ERR_FAIL_COND_V(!node || p_config.get_type() != Variant::OBJECT, ERR_INVALID_PARAMETER);
	MultiplayerSynchronizer *sync = Object::cast_to<MultiplayerSynchronizer>(p_config.get_validated_object());
	ERR_FAIL_COND_V(!sync, ERR_INVALID_PARAMETER);
	
	TrackedNode &tobj = _track(p_obj->get_instance_id());
	const ObjectID sid = sync->get_instance_id();
	tobj.synchronizers.insert(sid);
	sync_nodes.insert(sid);