Xamarin Forms Shell Navigation Failure with System.NullReferenceException - xamarin.forms

Question

This is a sample application attempting the Xamarin Forms navigation capability.
We have an event handler with this simple logic:
async void Button_Clicked(System.Object sender, System.EventArgs e)
{
await Shell.Current.GoToAsync(nameof(SchoolList));
}
We also have the route registered in App.xaml.cs as follows:
public partial class App : Application
{
public App()
{
InitializeComponent();
MainPage = new MainPage();
Routing.RegisterRoute(nameof(SchoolList), typeof(SchoolList));
}
However, when the corresponding button is clicked, we get the error below:
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+<>c.<ThrowAsync>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+<>c__DisplayClass2_0.<Post>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)
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.

Answer 1

It looks like you don’t have a Shell, so Shell.Current is null.
What Shell example did you follow?
MainPage = new MainPage(); means your app is pointing to a page of type MainPage. To use Shell (Route) navigation, it needs to be pointing to a Shell.
I would expect to see MainPage = new AppShell();.
There may be other details missing, so be sure to follow a working example, such as Xaminals.
The new AppShell line can be seen in https://github.com/xamarin/xamarin-forms-samples/blob/main/UserInterface/Xaminals/Xaminals/App.xaml.cs.
Note: Its possible to navigate in Xamarin Forms without having Shell, nor defining routes. (Personally, I dislike the Shell, so I don’t use it.) You can define MainPage as a NavigationPage (see its doc). Or you can simply set App.MainPage to different pages, to move between them, without a navigation stack.