Handle disconnect client

Hello, I have problem with the network UNET.
The problem is when the client just close the game but the error is occurs on the server.
Here the error I have :


I have implement OnClientError, OnServerError, OnClientDisconnect, OnServerDisconnect, but none of the method is called.

I am using Unity 5.4.2f, so my question is how I can handle those client from disconnecting?

Updates : Just made very simple networking game using Unity, and this error is also occurs. So how to repro this bugs is just play the network game, and close the client and the server will get this error.

This bugs does not happen on 5.5