При развертывании


System.NullReferenceException не обработано
  Message=NullReferenceException
  StackTrace:
       at wp7nag.MainViewModel.PushChannel_ChannelUriUpdated(Object sender, NotificationChannelUriEventArgs e)
       at Microsoft.Phone.Notification.HttpNotificationChannel.OnDescriptorUpdated(IntPtr blob, UInt32 blobSize)
       at Microsoft.Phone.Notification.HttpNotificationChannel.ChannelHandler(UInt32 eventType, IntPtr blob1, UInt32 int1, IntPtr blob2, UInt32 int2)
       at Microsoft.Phone.Notification.HttpNotificationChannel.Dispatch(Object threadContext)
       at System.Threading.ThreadPool.WorkItem.WaitCallback_Context(Object state)
       at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
       at System.Threading.ThreadPool.WorkItem.doWork(Object o)
       at System.Threading.Timer.ring()

подсвеченный код:
        void PushChannel_ChannelUriUpdated(object sender, NotificationChannelUriEventArgs e)
        {
            AppSettings settings = new AppSettings();
            CallWithDispatcher(() =>
            {
                settings.PushURLSetting = String.Format("Channel Uri Updated, new Received\n");
                Uri uri = new Uri((string)settings.URLwpNagSetting + "?cmd=14?&push=" + e.ChannelUri.ToString()); // , UriKind.Relative);
                WebClient webClient = new WebClient();
                webClient.Credentials = new NetworkCredential((string)settings.UserNameSetting, (string)settings.UserPasswordSetting);
                webClient.DownloadStringCompleted += OnRegisterPushCompleted;
                webClient.DownloadStringAsync(uri);
            });
        }


up 06.05.2013 :
Выяснил, что проблемы вызывается тем что между открытием канала и получением урла проходит относительно много времени. Закостылил пустым циклом:
            // Try to find the push channel.
            pushChannel = HttpNotificationChannel.Find(channelName);
            // If the channel was not found, then create a new connection to the push service.
            if (pushChannel == null)
            {
                pushChannel = new HttpNotificationChannel(channelName);
                // Register for all the events before attempting to open the channel.
                pushChannel.ChannelUriUpdated += new EventHandler<NotificationChannelUriEventArgs>(PushChannel_ChannelUriUpdated);
                pushChannel.ErrorOccurred += new EventHandler<NotificationChannelErrorEventArgs>(PushChannel_ErrorOccurred);
                pushChannel.Open();
                // Bind this new channel for Tile events.
                settings.PushURLSetting = String.Format("Push Channel Activated\n");
                while (pushChannel.ChannelUri == null)
                {
//                  Wait for receiving Channel URI
                }
//                DeviceRegister(pushChannel.ChannelUri.ToString());
            }
            else
            {