await x.ShowAsync(); shown in windows phone 8.1 app Erorr

private void Button_Click(object sender, RoutedEventArgs e)
       {
           try
           {
               int snooze;
               string audioSrc;
               int year = datepicker.Date.Year;
               int month = datepicker.Date.Month;
               int day = datepicker.Date.Day;
               int hour = timepicker.Time.Hours;
               int min = timepicker.Time.Minutes;
               int sec = timepicker.Time.Seconds;
               //   string audioSrc = alrm_sound.SelectionBoxItem.ToString();
               try
               {
                   snooze = Convert.ToInt16(CustomSnoozeTime.SelectionBoxItem.ToString());
               }
               catch
               {
                   snooze = 5;
               }
               try
               {
                   audioSrc = alrm_sound.SelectionBoxItem.ToString();
               }
               catch
               {
                   audioSrc = "Default";
               }
               DateTime myDate1 = new DateTime(year, month, day, hour, min, sec);

               DateTime myDate2 = DateTime.Now;
               TimeSpan myDateResult = new TimeSpan();
               myDateResult = myDate1 - myDate2;
               if (myDate2 > myDate1)
               {
                   var x = new MessageDialog("Invalid date or time");
                   await x.ShowAsync();
               }
               else
               {

                   string title = "Alarm!";
                   string message = alm_msg.Text;
                   string imgURL = "ms-appx:///Assets/Capture.PNG";

                   string toastXmlString =
                    "<toast><visual version='1'><binding template='toastImageAndText02'><text id='1'>"
                  + title + "</text><text id='2'>"
                       + message + "</text><image id='1' src='" + imgURL + "'/></binding></visual>\n" +
                        "<commands scenario=\"alarm\">\n" +
                           "<command id=\"snooze\"/>\n" +
                           "<command id=\"dismiss\"/>\n" +
                       "</commands>\n" +
                             "<audio src='ms-winsoundevent:Notification." + audioSrc + "'/>" +
                       "</toast>";

                   Windows.Data.Xml.Dom.XmlDocument toastDOM = new Windows.Data.Xml.Dom.XmlDocument();
                   toastDOM.LoadXml(toastXmlString);
                   var toastNotifier1 = Windows.UI.Notifications.ToastNotificationManager.CreateToastNotifier();

                   double x1 = myDateResult.TotalSeconds;
                   int customSnoozeSeconds = snooze * 60;

                   TimeSpan snoozeInterval = TimeSpan.FromSeconds(customSnoozeSeconds);

                   var customAlarmScheduledToast = new Windows.UI.Notifications.ScheduledToastNotification(toastDOM, DateTime.Now.AddSeconds(x1), snoozeInterval, 0);

                   toastNotifier1.AddToSchedule(customAlarmScheduledToast);
                   var x = new MessageDialog("Alarm Set!");
                   await x.ShowAsync();
               }

           }
           catch
           { }
       }