PushModalAsync not working from UploadStringAsync

after end of UploadStringAsync (in UploadStringCompleted) i can't get to switch to another page
Xamarin.Forms.Device.BeginInvokeOnMainThread(() =>
{
Navigation.PushModalAsync(new ConfirmRandomActivity(aaa));
});
this is problem on WP, on Android I don't have to call it from UploadStringCompleted. there is normally function waiting for end of async upload.
shoud I use something else?

thanks