how open menu page after login throw webervice

hi, I'm new in xamarin developing, I have such problem - I have webervice method called on user login action, and in handler onmethodcomplete I try open new page writing this.Navigation.PushAsync(new MenuPage()), but nothing happens, even page disappeares, in logcat I cath eroor - Xamarin.Forms.InvalidNavigationException Can't pop modal without any modals
pushed
Xamarin.Forms.InvalidNavigationException Can't pop modal without any modals
pushed. Can anyone suggest me or navigate to true way in this case ? may be there are some advises or some examples ? Thank everyone, and sorry for my English. I'm using Visual Studio 2012 and latest xamarin plugins version, debugging application on android device.