### 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)

}
		
		if (sync->is_visible_to(p_peer)) {
			is_visible = true;
			break;
		}
		is_visible = false;
	}
	
	HashSet<int> to_spawn;
	HashSet<int> to_despawn;
	if (p_peer) {
		ERR_FAIL_COND_V(!peers_info.has(p_peer), ERR_INVALID_PARAMETER);
		if (is_visible == peers_info[p_peer].spawn_nodes.has(p_oid)) {
			return OK;
		}
		if (is_visible) {
			to_spawn.insert(p_peer);
		} else {
			to_despawn.insert(p_peer);
		}
	} else {
		
		for (const KeyValue<int, PeerInfo> &E : peers_info) {
			if (is_visible) {
				
				if (E.value.spawn_nodes.has(p_oid)) {
					
					continue;
				}