Fix #401: ArgumentNullException after resetting REPL
Fix #401: ArgumentNullException after resetting REPL

Add null check for exception in OnError websocket handler.

karthiknadig/RTVS,AlexanderSher/RTVS,karthiknadig/RTVS,karthiknadig/RTVS,MikhailArkhipov/RTVS,MikhailArkhipov/RTVS,MikhailArkhipov/RTVS,AlexanderSher/RTVS,MikhailArkhipov/RTVS,AlexanderSher/RTVS,AlexanderSher/RTVS,AlexanderSher/RTVS,AlexanderSher/RTVS,AlexanderSher/RTVS,karthiknadig/RTVS,MikhailArkhipov/RTVS,karthiknadig/RTVS,MikhailArkhipov/RTVS,karthiknadig/RTVS,MikhailArkhipov/RTVS,karthiknadig/RTVS