how solev erorr in xamarinForm and PageNavigation : when i use Xamarin form  Navigation page get me an error and show me a dialog 
[enter image description here][1]


and my code is here
[enter image description here][2]


  [1]: https://i.stack.imgur.com/owVrL.png
  [2]: https://i.stack.imgur.com/YmOUD.png

please help me!!!!