title: Why my app is crashing when OnBackPressed is called?
body: <p>I'm trying to execute some javascript(webview Eval) when back button(OnBackPressed) is pressed but the app crashs. Can someone figure out what is happening?</p>
  
  <p>MainActivity.cs</p>
  
  <pre><code>public class MainActivity : global::Xamarin.Forms.Platform.Android.FormsAppCompatActivity
  {
      protected override void OnCreate(Bundle savedInstanceState)
      {
          /*
       some code
      */
          base.OnCreate(savedInstanceState);
          global::Xamarin.Forms.Forms.Init(this, savedInstanceState);
          LoadApplication(new App());
      }
  
      public override void OnBackPressed()
      {
          MainPage.ExecJS();
  }
  }
  </code></pre>
  
  <p>MainPage.xaml.cs</p>
  
  <pre><code> public partial class MainPage : ContentPage
   {
      public static double ScreenWidth { get; set; }
      public static double ScreenHeight { get; set; }
      public static WebView webnav;
      StackLayout parent = null;
      public MainPage()
      {
          InitializeComponent();
          WebView webnav = new WebView
          {
              HeightRequest = 1000,
              WidthRequest = 1000,
              Source = "https://www.comprarmusicas.com.br/mobile/index.php"
          };
          parent = new StackLayout();
          parent.Children.Add(webnav);
          Content = parent;
      }
  
       public static void ExecJS()
      {
          string jsCode = "alert('ok');";
          webnav.Eval(jsCode);
      }
   }
  </code></pre>