To reproduce:
1. Open attached project (server)
2. Create a new project in a different editor window (client)
3. Paste this into a script and add it to an object:

public class GameController : MonoBehaviour
{
NetworkClient _net;

private void Start()
{
_net = new NetworkClient();
_net.Connect( "127.0.0.1", 21999 );
}
}

4. Enter play mode on the server project
5. Enter play mode on the client project
6. After the client project connects to the server project, exit play mode on the client project
Notice that errors appear in the console of the server project.

Reproducible on: 5.5.0b4, 5.5.0b3
Not reproducible on: 5.4.0f2