NullReferenceException in WebSocket.get_IsAlive () after error in socket

This exception we have everyday after websocket-server restart

System.NullReferenceException: Object reference not set to an instance of an object
at WebSocketSharp.WebSocket.Ping (System.Byte[] frame, Int32 timeout) [0x00000] in <filename unknown>:0
at WebSocketSharp.WebSocket.Ping () [0x00000] in <filename unknown>:0
at WebSocketSharp.WebSocket.get_IsAlive () [0x00000] in <filename unknown>:0
at WebSocketController.Stop () [0x00000] in <filename unknown>:0
at WebSocketController.OnError (System.Object sender, WebSocketSharp.ErrorEventArgs errorEventArgs) [0x00000] in <filename unknown>:0
at (wrapper delegate-invoke) System.EventHandler`1<WebSocketSharp.ErrorEventArgs>:invoke_void__this___object_TEventArgs (object,WebSocketSharp.ErrorEventArgs)
at WebSocketSharp.Ext.Emit[ErrorEventArgs] (System.EventHandler`1 eventHandler, System.Object sender, WebSocketSharp.ErrorEventArgs e) [0x00000] in <filename unknown>:0
at WebSocketSharp.WebSocket.error (System.String message) [0x00000] in <filename unknown>:0
at WebSocketSharp.WebSocket.acceptException (System.Exception exception, System.String reason) [0x00000] in <filename unknown>:0
at WebSocketSharp.WebSocket+<>c__DisplayClass18.<startReceiving>b__17 (System.Exception ex) [0x00000] in <filename unknown>:0
at WebSocketSharp.Ext+<>c__DisplayClass6.<ReadBytesAsync>b__5 (IAsyncResult ar) [0x00000] in <filename unknown>:0

Where WebSocketController.Stop() is

if (ws != null && ws.IsAlive)
{
    ws.Close(CloseStatusCode.Normal, "Shutdown");
}

Start with WebSocket.Ping and WebSocket.get_IsAlive in the stack trace, then inspect how WebSocketController.Stop calls IsAlive after an error. Reproduce the websocket-server restart sequence and verify that stopping the controller no longer raises NullReferenceException.