Xamarin.Forms.Android: java.lang.IllegalStateException: Fragment hat keine Ansicht

Ich entwickle eine Xamarin.Forms-App. Es funktioniert gut, stürzt aber immer wieder ab, wenn der Benutzer es nach dem Öffnen einiger anderer Apps aus dem Hintergrund zurückbringt. Ich bin neu in Xamarin und nicht sicher, wie ich dieses Problem beheben soll. Dieses Problem tritt unter iOS bisher nicht auf.

SplashActivity.cs:

using System.Threading.Tasks;
using Android.App;
using Android.Content;
using Android.OS;
using Android.Support.V7.App;
using Android.Util;

namespace MyApp.Droid
{
    [Activity(Theme = "@style/MyTheme.Splash", MainLauncher = true, NoHistory = true)]
    public class SplashActivity : AppCompatActivity
    {
        static readonly string TAG = "X:" + typeof(SplashActivity).Name;

        protected override void OnCreate(Bundle savedInstanceState) //  PersistableBundle persistentState
        {
            base.OnCreate(savedInstanceState); // persistentState
            Log.Debug(TAG, "SplashActivity.OnCreate");
        }

        protected override void OnResume()
        {
            base.OnResume();

            Task startupWork = new Task(() =>
                                        {
                                            Log.Debug(TAG, "Performing some startup work that takes a bit of time.");
                                            Task.Delay(5000); // Simulate a bit of startup work.
                                            Log.Debug(TAG, "Working in the background - important stuff.");
                                        });

            startupWork.ContinueWith(t =>
                                     {
                                         Log.Debug(TAG, "Work is finished - start Activity1.");
                                         StartActivity(new Intent(Application.Context, typeof(MainActivity)));
                                     }, TaskScheduler.FromCurrentSynchronizationContext());

            startupWork.Start();
        }
    }
}

MainActivity.cs:

using System;

using Android.App;
using Android.Content;
using Android.Content.PM;
using Android.Runtime;
using Android.Views;
using Android.Widget;
using Android.OS;

using HockeyApp.Android;
using HockeyApp.Android.Metrics;

using Xamarin.Android.Net; // Keep to make TLS 1.2 work

namespace MyApp.Droid
{
    [Activity(Label = "EventingVolunteers.Droid", Icon = "@drawable/icon", Theme = "@style/MyTheme", ConfigurationChanges = ConfigChanges.ScreenSize | ConfigChanges.Orientation)]
    public class MainActivity : global::Xamarin.Forms.Platform.Android.FormsAppCompatActivity
    {
        protected override void OnCreate(Bundle savedInstanceState)
        {
            TabLayoutResource = Resource.Layout.Tabbar;
            ToolbarResource = Resource.Layout.Toolbar;

            base.OnCreate(savedInstanceState);

            global::Xamarin.Forms.Forms.Init(this, savedInstanceState);

            LoadApplication(new App());

            // HockeyApp
            CrashManager.Register(this, Constants.HockeyAppID_Android);
            MetricsManager.Register(this, Application, Constants.HockeyAppID_Android);

            // Push Notifications
            RegisterForGCM();
        }

        private void RegisterForGCM()
        {
            string senders = Constants.GoogleConsoleProjectId;
            Intent intent = new Intent("com.google.android.c2dm.intent.REGISTER");
            intent.SetPackage("com.google.android.gsf");
            intent.PutExtra("app", PendingIntent.GetBroadcast(this, 0, new Intent(), 0));
            intent.PutExtra("sender", senders);
            StartService(intent);
        }

        protected override void OnPause()
        {
            base.OnPause();
        }

        protected override void OnResume()
        {
            base.OnResume();
        }

        protected override void OnDestroy()
        {
            base.OnDestroy();
        }
    }
}

Fehlermeldung:

java.lang.IllegalStateException: Fragment does not have a view
android.support.v4.app.Fragment$1.onFindViewById()Fragment.java:1933
android.support.v4.app.FragmentManagerImpl.moveToState()FragmentManager.java:1057
android.support.v4.app.FragmentManagerImpl.moveToState()FragmentManager.java:1252
android.support.v4.app.FragmentManagerImpl.moveToState()FragmentManager.java:1234
android.support.v4.app.FragmentManagerImpl.dispatchActivityCreated()FragmentManager.java:2046
android.support.v4.app.Fragment.performActivityCreated()Fragment.java:1989
android.support.v4.app.FragmentManagerImpl.moveToState()FragmentManager.java:1092
android.support.v4.app.FragmentManagerImpl.moveToState()FragmentManager.java:1252
android.support.v4.app.FragmentManagerImpl.moveToState()FragmentManager.java:1234
android.support.v4.app.FragmentManagerImpl.dispatchActivityCreated()FragmentManager.java:2046
android.support.v4.app.FragmentController.dispatchActivityCreated()FragmentController.java:174
android.support.v4.app.FragmentActivity.onStart()FragmentActivity.java:598
md5b60ffeb829f638581ab2bb9b1a7f4f3f.FormsAppCompatActivity.n_onStart(Native Method)
md5b60ffeb829f638581ab2bb9b1a7f4f3f.FormsAppCompatActivity.onStart()FormsAppCompatActivity.java:120
android.app.Instrumentation.callActivityOnStart()Instrumentation.java:1260
android.app.Activity.performStart()Activity.java:6261
android.app.ActivityThread.performLaunchActivity()ActivityThread.java:2389
android.app.ActivityThread.handleLaunchActivity()ActivityThread.java:2490
at android.app.ActivityThread.-wrap11(ActivityThread.java)
android.app.ActivityThread$H.handleMessage()ActivityThread.java:1354
android.os.Handler.dispatchMessage()Handler.java:102
android.os.Looper.loop()Looper.java:148
android.app.ActivityThread.main()ActivityThread.java:5456
java.lang.reflect.Method.invoke(Native Method)
com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run()ZygoteInit.java:728
com.android.internal.os.ZygoteInit.main()ZygoteInit.java:618
Xamarin caused by: Java.Lang.IllegalStateException: Fragment does not have a view
System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
Java.Interop.JniEnvironment.InstanceMethods.CallNonvirtualVoidMethod(JniObjectReference instance, JniObjectReference type, JniMethodInfo method, JniArgumentValue* args)
Java.Interop.JniPeerMembers.JniInstanceMethods.InvokeVirtualVoidMethod(string encodedMember, IJavaPeerable self, JniArgumentValue* parameters)
Android.App.Activity.OnStart()
Xamarin.Forms.Platform.Android.FormsAppCompatActivity.OnStart()
Android.App.Activity.n_OnStart(IntPtr jnienv, IntPtr native__this)
at (wrapper dynamic-method) System.Object:23d8bc20-60cf-4cac-a339-6cc6722e8667 (intptr,intptr)
--- End of managed Java.Lang.IllegalStateException stack trace ---
java.lang.IllegalStateException: Fragment does not have a view
android.support.v4.app.Fragment$1.onFindViewById()Fragment.java:1933
android.support.v4.app.FragmentManagerImpl.moveToState()FragmentManager.java:1057
android.support.v4.app.FragmentManagerImpl.moveToState()FragmentManager.java:1252
android.support.v4.app.FragmentManagerImpl.moveToState()FragmentManager.java:1234
android.support.v4.app.FragmentManagerImpl.dispatchActivityCreated()FragmentManager.java:2046
android.support.v4.app.Fragment.performActivityCreated()Fragment.java:1989
android.support.v4.app.FragmentManagerImpl.moveToState()FragmentManager.java:1092
android.support.v4.app.FragmentManagerImpl.moveToState()FragmentManager.java:1252
android.support.v4.app.FragmentManagerImpl.moveToState()FragmentManager.java:1234
android.support.v4.app.FragmentManagerImpl.dispatchActivityCreated()FragmentManager.java:2046
android.support.v4.app.FragmentController.dispatchActivityCreated()FragmentController.java:174
android.support.v4.app.FragmentActivity.onStart()FragmentActivity.java:598
md5b60ffeb829f638581ab2bb9b1a7f4f3f.FormsAppCompatActivity.n_onStart(Native Method)
md5b60ffeb829f638581ab2bb9b1a7f4f3f.FormsAppCompatActivity.onStart()FormsAppCompatActivity.java:120
android.app.Instrumentation.callActivityOnStart()Instrumentation.java:1260
android.app.Activity.performStart()Activity.java:6261
android.app.ActivityThread.performLaunchActivity()ActivityThread.java:2389
android.app.ActivityThread.handleLaunchActivity()ActivityThread.java:2490
at android.app.ActivityThread.-wrap11(ActivityThread.java)
android.app.ActivityThread$H.handleMessage()ActivityThread.java:1354
android.os.Handler.dispatchMessage()Handler.java:102
android.os.Looper.loop()Looper.java:148
android.app.ActivityThread.main()ActivityThread.java:5456
java.lang.reflect.Method.invoke(Native Method)
com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run()ZygoteInit.java:728
com.android.internal.os.ZygoteInit.main()ZygoteInit.java:618
--- End of managed Java.Lang.IllegalStateException stack trace ---
java.lang.IllegalStateException: Fragment does not have a view
android.support.v4.app.Fragment$1.onFindViewById()Fragment.java:1933
android.support.v4.app.FragmentManagerImpl.moveToState()FragmentManager.java:1057
android.support.v4.app.FragmentManagerImpl.moveToState()FragmentManager.java:1252
android.support.v4.app.FragmentManagerImpl.moveToState()FragmentManager.java:1234
android.support.v4.app.FragmentManagerImpl.dispatchActivityCreated()FragmentManager.java:2046
android.support.v4.app.Fragment.performActivityCreated()Fragment.java:1989
android.support.v4.app.FragmentManagerImpl.moveToState()FragmentManager.java:1092
android.support.v4.app.FragmentManagerImpl.moveToState()FragmentManager.java:1252
android.support.v4.app.FragmentManagerImpl.moveToState()FragmentManager.java:1234
android.support.v4.app.FragmentManagerImpl.dispatchActivityCreated()FragmentManager.java:2046
android.support.v4.app.FragmentController.dispatchActivityCreated()FragmentController.java:174
android.support.v4.app.FragmentActivity.onStart()FragmentActivity.java:598
md5b60ffeb829f638581ab2bb9b1a7f4f3f.FormsAppCompatActivity.n_onStart(Native Method)
md5b60ffeb829f638581ab2bb9b1a7f4f3f.FormsAppCompatActivity.onStart()FormsAppCompatActivity.java:120
android.app.Instrumentation.callActivityOnStart()Instrumentation.java:1260
android.app.Activity.performStart()Activity.java:6261
android.app.ActivityThread.performLaunchActivity()ActivityThread.java:2389
android.app.ActivityThread.handleLaunchActivity()ActivityThread.java:2490
at android.app.ActivityThread.-wrap11(ActivityThread.java)
android.app.ActivityThread$H.handleMessage()ActivityThread.java:1354
android.os.Handler.dispatchMessage()Handler.java:102
android.os.Looper.loop()Looper.java:148
android.app.ActivityThread.main()ActivityThread.java:5456
java.lang.reflect.Method.invoke(Native Method)
com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run()ZygoteInit.java:728
com.android.internal.os.ZygoteInit.main()ZygoteInit.java:618

App.xaml.cs:

using System;
using System.Linq;
using Amazon;
using Xamarin.Forms;

namespace MyApp
{
    public partial class App : Application
    {
        public static string AppName { get { return "MyApp"; } }

        public static User User { get; set; }

        public static UserManager UserManager { get; private set; }

        public static VolunteerManager VolunteerManager { get; private set; }

        public static SignupManager SignupManager { get; private set; }

        static NavigationPage NavPage;

        public static bool IsLoggedIn
        {
            get
            {
                if (User != null)
                    return !string.IsNullOrWhiteSpace(User.Email);
                else
                    return false;
            }
        }

        public static Action SuccessfulLoginAction
        {
            get
            {
                return new Action(() =>
                {
                    NavPage.Navigation.PopModalAsync();

                    if (IsLoggedIn)
                    {
                        // Update RestService with new token
                        UserManager = new UserManager(new RestService());
                        VolunteerManager = new VolunteerManager(new RestService());
                        SignupManager = new SignupManager(new RestService());

                        NavPage.Navigation.InsertPageBefore(new VolunteerDashboardPage(), NavPage.Navigation.NavigationStack.First());
                        NavPage.Navigation.PopToRootAsync();
                    }
                });
            }
        }

        public static Action LogoutAction
        {
            get
            {
                return new Action(() =>
                {
                    DependencyService.Get<ICredentialsService>().DeleteCredentials();
                    App.User = null;
                    // NavPage.Navigation.PopModalAsync();
                    NavPage.Navigation.InsertPageBefore(new LoginPage(), NavPage.Navigation.NavigationStack.First());
                    NavPage.Navigation.PopToRootAsync();
                });
            }
        }

        public App()
        {
            User = new User();
            UserManager = new UserManager(new RestService());
            VolunteerManager = new VolunteerManager(new RestService());
            SignupManager = new SignupManager(new RestService());

            // AWS SDK
            var loggingConfig = AWSConfigs.LoggingConfig;
            loggingConfig.LogMetrics = true;
            loggingConfig.LogResponses = ResponseLoggingOption.Always;
            loggingConfig.LogMetricsFormat = LogMetricsFormatOption.JSON;
            loggingConfig.LogTo = LoggingOptions.SystemDiagnostics;

            //AWSConfigs.AWSRegion = "us-east-1";

            // Check if login needed
            if (DependencyService.Get<ICredentialsService>().DoCredentialsExist())
            {
                NavPage = new NavigationPage(new VolunteerDashboardPage());
                MainPage = NavPage;
                // MainPage = new VolunteerDashboardPage();
            } else {
                NavPage = new NavigationPage(new LoginPage());
                MainPage = NavPage;
            }
        }

        protected override void OnStart()
        {
            // Handle when your app starts
        }

        protected override void OnSleep()
        {
            // Handle when your app sleeps
        }

        protected override void OnResume()
        {
            // Handle when your app resumes
        }
    }
}