multiplayer-community-contributions: NullReferenceException when using SteamP2P Transport

I am using the SteamP2P Transport and have a simple setup for a lobby in unity. When trying to join a hosted game as a client, after a few seconds a NullReferenceException gets thrown in the NetworkManager.cs, at line 1144: MessagingSystem.ProcessIncomingMessageQueue();

Then the NetworkManager.Singleton.OnClientDisconnectCallback is called, against what the NetworkManager.Singleton.OnClientConnectedCallback never gets called (so the client never connects properly).

Here are two threads, where people describe the similar problems:

1. https://answers.unity.com/questions/1869491/messagingsystemprocessincomingmessagequeue-nullref.html?childToView=1870613
2. forum.unity.com/threads/anyone-have-sample-mlapi-with-steam-matchmaking.1098304/

I am using:

• Unity 2020.3.20f1
• Netcode for GameObjects 1.0.0-pre.3
• Multiplayer Tools 1.0.0-pre.2
• SteamP2P Transport for Netcode for GameObjects 2.0.0