Fix NullReferenceException in RHost when RSession does not have a _hostClientApp.
Fix NullReferenceException in RHost when RSession does not have a _hostClientApp.

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