title: Xamarin Forms Shell Navigation Failure with System.NullReferenceException
body: <p>This is a sample application attempting the Xamarin Forms navigation capability.
  We have an event handler with this simple logic:</p>
  <pre><code>async void Button_Clicked(System.Object sender, System.EventArgs e)
  {
      await Shell.Current.GoToAsync(nameof(SchoolList));
  }
  </code></pre>
  <p>We also have the route registered in App.xaml.cs as follows:</p>
  <pre><code>public partial class App : Application
      {
          public App()
          {
              InitializeComponent();
  
              MainPage = new MainPage();
              Routing.RegisterRoute(nameof(SchoolList), typeof(SchoolList));
          }
  </code></pre>
  <p>However, when the corresponding button is clicked, we get the error below:</p>
  <pre><code>System.NullReferenceException: Object reference not set to an instance of an object.
    at cross_app1.MainPage.Button_Clicked (System.Object sender, System.EventArgs e) [0x0000f] in /Users/klaus.nji/Projects/cross-app1/cross-app1/MainPage.xaml.cs:18
    at System.Runtime.CompilerServices.AsyncMethodBuilderCore+&lt;&gt;c.&lt;ThrowAsync&gt;b__7_0 (System.Object state) [0x00000] in /Users/builder/jenkins/workspace/archive-mono/2020-02/android/release/mcs/class/referencesource/mscorlib/system/runtime/compilerservices/AsyncMethodBuilder.cs:1021
    at Android.App.SyncContext+&lt;&gt;c__DisplayClass2_0.&lt;Post&gt;b__0 () [0x00000] in /Users/builder/azdo/_work/1/s/xamarin-android/src/Mono.Android/Android.App/SyncContext.cs:36
    at Java.Lang.Thread+RunnableImplementor.Run () [0x00008] in /Users/builder/azdo/_work/1/s/xamarin-android/src/Mono.Android/Java.Lang/Thread.cs:36
    at Java.Lang.IRunnableInvoker.n_Run (System.IntPtr jnienv, System.IntPtr native__this) [0x00008] in /Users/builder/azdo/_work/1/s/xamarin-android/src/Mono.Android/obj/Release/monoandroid10/android-30/mcw/Java.Lang.IRunnable.cs:84
    at at (wrapper dynamic-method) Android.Runtime.DynamicMethodNameCounter.39(intptr,intptr)
  </code></pre>
  <p>The system is Mac Catalina v10.15.7, IDE is Visual Studio 2019 for Mac and I have the Andriod SDK installed and able to see other aspects of the app.</p>