message: Add null check for app in HandleSubscribe
diff: @@ -68,11 +68,10 @@ void GetAppServiceDataRequest::Run() {
  void GetAppServiceDataRequest::HandleSubscribe() {
  std::string service_type =
  (*message_)[strings::msg_params][strings::service_type].asString();
  - if ((*message_)[strings::msg_params].keyExists(strings::subscribe)) {
  + ApplicationSharedPtr app = application_manager_.application(connection_key());
  + if (app && (*message_)[strings::msg_params].keyExists(strings::subscribe)) {
  bool subscribe =
  (*message_)[strings::msg_params][strings::subscribe].asBool();
  - ApplicationSharedPtr app =
  - application_manager_.application(connection_key());
  auto& ext = AppServiceAppExtension::ExtractASExtension(*app);
  if (subscribe) {
  ext.SubscribeToAppService(service_type);
  