如果在不使用MessageDialog加载页面时隐藏了一些控件，则应用程序崩溃

我有一个应用程序，其中有一个设置，允许用户停止应用程序对其位置的访问。这存储在Windows.Storage.ApplicationData.Current.RoamingSettings.Values["location"]中。如果定位服务+此设置允许访问，则我加载一个打开地图的页面。如果设置允许访问，并且定位服务处于关闭状态，则在加载页面时会显示一条消息并隐藏一些控件。如果设置关闭，那么我只想隐藏控件而不显示任何消息。

protected  override void OnNavigatedTo(NavigationEventArgs e)
    {
         .....
        // MUST ENABLE THE LOCATION CAPABILITY!!!
          var locator = new Geolocator();
          locator.DesiredAccuracyInMeters = 50;
          locator.ReportInterval = (uint)TimeSpan.FromSeconds(15).TotalMilliseconds;
          setloc(locator);
          this.navigationHelper.OnNavigatedTo(e);
    }
    public async void setloc(Geolocator locator)
    {
        if (locator.LocationStatus != PositionStatus.Disabled && (bool)Windows.Storage.ApplicationData.Current.RoamingSettings.Values["location"]==true)
        {
            var position = await locator.GetGeopositionAsync();
            await MyMap.TrySetViewAsync(position.Coordinate.Point, 16D);
            ....
            return;
        }
        else if (locator.LocationStatus == PositionStatus.Disabled && (bool)Windows.Storage.ApplicationData.Current.RoamingSettings.Values["location"] == true)
        {
            MessageDialog msgbox = new MessageDialog("Location Services are turned off. Please turn them on to save Location while saving a Tip", "Location Unavailable");
            await msgbox.ShowAsync();
            savebutton.Visibility = Visibility.Collapsed;
            myMapBlock.Visibility = Visibility.Collapsed;
            return;
        }
       ***// MessageDialog msgbox1 = new MessageDialog("Location Services are turned off. Please turn them on to save Location while saving a Tip", "Location Unavailable");
       // await msgbox1.ShowAsync();***
       savebutton.Visibility = Visibility.Collapsed;
       myMapBlock.Visibility = Visibility.Collapsed;
    }

当该设置打开(true(时一切都很好，但当它关闭(false(时，会发生一些奇怪的事情。上面的代码不起作用。它会导致应用程序崩溃，但当我取消注释代码中***内的部分时，会显示消息，页面也会正确加载。如果我只是试图在不使用MessageDialog的情况下隐藏myMapBlock和保存按钮，它就会崩溃。我想在不使用MessageDialog的情况下隐藏控件。我该怎么做？

setloc(locator);

await Windows.UI.Core.CoreWindow.GetForCurrentThread().Dispatcher.RunAsync(Windows.UI.Core.CoreDispatcherPriority.Normal, async () => { await setloc(locator); });