I looked into this and couldn't find anything that sets the Server.ServerSocket _server field of Yak.cs:19.
NullReferenceException: Object reference not set to an instance of an object
FishNet.Transporting.Yak.Yak.StartServer () (at Assets/FishNet/Runtime/Plugins/Yak/Yak.cs:262)
FishNet.Transporting.Yak.Yak.StartConnection (System.Boolean server) (at Assets/FishNet/Runtime/Plugins/Yak/Yak.cs:218)
FishNet.Transporting.Multipass.Multipass.StartServer (System.Int32 index) (at Assets/FishNet/Runtime/Transporting/Transports/Multipass/Multipass.cs:935)
FishNet.Transporting.Multipass.Multipass.StartConnection (System.Boolean server, System.Int32 index) (at Assets/FishNet/Runtime/Transporting/Transports/Multipass/Multipass.cs:816)
FishNet.Transporting.Multipass.Multipass.StartConnection (System.Boolean server) (at Assets/FishNet/Runtime/Transporting/Transports/Multipass/Multipass.cs:792)
Thank you!