### 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)) {
				return true;
			}
		}
		return false; 
	}
}
Error SceneReplicationInterface::_update_sync_visibility(int p_peer, MultiplayerSynchronizer *p_sync) {
	ERR_FAIL_COND_V(!p_sync, ERR_BUG);
	if (!multiplayer->has_multiplayer_peer() || !p_sync->is_multiplayer_authority() || p_peer == multiplayer->get_unique_id()) {
		return OK;
	}
	const ObjectID &sid = p_sync->get_instance_id();
	bool is_visible = p_sync->is_visible_to(p_peer);
	if (p_peer == 0) {
		for (KeyValue<int, PeerInfo> &E : peers_info) {
			
			bool is_visible_to_peer = is_visible || p_sync->is_visible_to(E.key);
			if (is_visible_to_peer == E.value.sync_nodes.has(sid)) {
				continue;
			}
			if (is_visible_to_peer) {
				E.value.sync_nodes.insert(sid);
			} else {
				E.value.sync_nodes.erase(sid);
				E.value.last_watch_usecs.erase(sid);
			}