A WebView that is on a NavigationPage, if I navigate back the Android Player crash

Hello
I have a ContentPage on a NavigationPage, on that ContentPage I have a Button if I click this, I navigate to a second ContentPage with a WebView on it. Till here everything works fine but if I click the Android back button or the back button in the actionbar the Android Player crashed. The Android Player is running on API19 and the Xamarin.Forms version is 1.2.3.0 that I am working on. Anybody else have this problem, do I something wrong?

Here my Example Code

App.cs

namespace WebViewTest
{
    public class App
    {
        public static Page GetMainPage ()
        {   
            return new NavigationPage (new StartPage ());
        }
    }
}

StartPage.xaml

<?xml version="1.0" encoding="UTF-8"?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms" xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml" x:Class="WebViewTest.Views.StartPage">
    <ContentPage.Content>
        <StackLayout>
            <Button Text="Go to WebView" Clicked="OnClicked" />
        </StackLayout>
    </ContentPage.Content>
</ContentPage>

StartPage.xaml.cs

namespace WebViewTest.Views
{   
    public partial class StartPage : ContentPage
    {   
        public StartPage ()
        {
            InitializeComponent ();
        }

        async public void OnClicked(object sender, EventArgs e)
        {
            await Navigation.PushAsync (new WebViewPage ());
        }
    }
}

WebViewPage.xaml

<?xml version="1.0" encoding="UTF-8"?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms" xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml" x:Class="WebViewTest.Views.WebViewPage">
    <ContentPage.Content>
        <WebView Source="http://www.google.ch" />
    </ContentPage.Content>
</ContentPage>