private void MainPage_BackRequested(object sender, BackRequestedEventArgs e)
 {
     if (MainWeb.CanGoBack)
     {
         MainWeb.GoBack();
         e.Handled = true;
     }
     else
     {
     }
 }
 //method is called when back button is pressed
 private void Backbtn_Click(object sender, RoutedEventArgs e)
 {
     try
     {
         MainWeb.GoBack();
     }
     catch (Exception)
     {
         Backbtn.IsEnabled = false;
     }
 }