System.NullReferenceException: Object reference not set to an instance of an object When changing ViewPager

Christopher Drosos 1 Reputation point
2021-08-14T12:37:30.06+00:00

I'm getting this error a lot (2% of users in various android versions 7.x,8.x,9.x,10.x and devices)

FragmentOddEvenResults.ShowDrawsAsync (System.Boolean isDeleteHistoryChecked, System.Int32 timeFromPosition, System.Int32 timeToPosition)  
System.NullReferenceException: Object reference not set to an instance of an object  

Main thread  
FragmentOddEvenResults.ShowDrawsAsync (System.Boolean isDeleteHistoryChecked, System.Int32 timeFromPosition, System.Int32 timeToPosition)  
FragmentOddEvenResults.<OnCreateView>b__53_4 (System.Object <p0>, System.EventArgs <p1>)  
AsyncMethodBuilderCore+<>c.<ThrowAsync>b__7_0 (System.Object state)  
SyncContext+<>c__DisplayClass2_0.<Post>b__0 ()  
Thread+RunnableImplementor.Run ()  
IRunnableInvoker.n_Run (System.IntPtr jnienv, System.IntPtr native__this)  
(wrapper dynamic-method) Android.Runtime.DynamicMethodNameCounter.45(intptr,intptr)  

This happens when a user clicks a button from the app menu in order to change the current view according to the Appcenter Diagnostics Event:

123255-code.jpg

For changing the View i use Viewpager in this way:

//The menu code  
public bool OnNavigationItemSelected(IMenuItem item)  
        {  
            // Cancel any reamining background task  
            ApplicationState.DisposeCancellationTokenSource();  

            if (item == null)  
                return false;  

            var id = item.ItemId;  
            var currentTab = nameof(Resource.Id.Nav_DrawSearch);  
            var drawer = FindViewById<DrawerLayout>(Resource.Id.drawer_layout);  

            if (id == Resource.Id.Nav_DrawSearch)  
            {  
                currentTab = nameof(Resource.Id.Nav_DrawSearch);  
                SetCurrentViewpager(PagerAdapterTypesEnum.Search);  
                CurrentTabLayout.GetTabAt(0).Select();  
            }  
            else if (id == Resource.Id.Nav_DrawsPerFrequency)  
            {  
                currentTab = nameof(Resource.Id.Nav_DrawsPerFrequency);  
                SetCurrentViewpager(PagerAdapterTypesEnum.Search);  
                CurrentTabLayout.GetTabAt(1).Select();  
            }  
            else if (id == Resource.Id.Nav_DrawsPredictions)  
            {  
                currentTab = nameof(Resource.Id.Nav_DrawsPredictions);  
                SetCurrentViewpager(PagerAdapterTypesEnum.Search);  
                CurrentTabLayout.GetTabAt(2).Select();  
            }  
            else if (id == Resource.Id.Nav_OddEvenResults)  
            {  
                currentTab = nameof(Resource.Id.Nav_OddEvenResults);  
                SetCurrentViewpager(PagerAdapterTypesEnum.OddEven);  
                CurrentTabLayout.GetTabAt(0).Select();  
            }  
            else if (id == Resource.Id.Nav_OddEvenDelays)  
            {  
                currentTab = nameof(Resource.Id.Nav_OddEvenDelays);  
                SetCurrentViewpager(PagerAdapterTypesEnum.OddEven);  
                CurrentTabLayout.GetTabAt(1).Select();  
            }  
            else if (id == Resource.Id.Nav_OddEvenNotifications)  
            {  
                currentTab = nameof(Resource.Id.Nav_OddEvenNotifications);  
                SetCurrentViewpager(PagerAdapterTypesEnum.OddEven);  
                CurrentTabLayout.GetTabAt(2).Select();  
            }  
            else if (id == Resource.Id.Nav_ColumnsStatistics)  
            {  
                currentTab = nameof(Resource.Id.Nav_ColumnsStatistics);  
                SetCurrentViewpager(PagerAdapterTypesEnum.Columns);  
                CurrentTabLayout.GetTabAt(0).Select();  
            }  
            else if (id == Resource.Id.Nav_ColumnsResultsPerNumbers)  
            {  
                currentTab = nameof(Resource.Id.Nav_ColumnsResultsPerNumbers);  
                SetCurrentViewpager(PagerAdapterTypesEnum.Columns);  
                CurrentTabLayout.GetTabAt(1).Select();  
            }  
            else if (id == Resource.Id.Nav_Settings)  
            {  
                currentTab = nameof(Resource.Id.Nav_Settings);  
                SetCurrentViewpager(PagerAdapterTypesEnum.Settings);  
            }  
            else if (id == Resource.Id.Nav_Info)  
            {  
                currentTab = nameof(Resource.Id.Nav_Info);  
                SetCurrentViewpager(PagerAdapterTypesEnum.Information);  
            }  
            else if (id == Resource.Id.Nav_Rate)  
            {  
                currentTab = nameof(Resource.Id.Nav_Rate);  
                RequestReview();  
            }  
            else if (id == Resource.Id.Nav_Share)  
            {  
                currentTab = nameof(Resource.Id.Nav_Share);  
                var appId = AppInfo.PackageName;  
                var url = $"https://play.google.com/store/apps/details?id={appId}";  

                ShareUri(url, GetString(Resource.String.app_slogan));  
            }  

            Microsoft.AppCenter.Analytics.Analytics.TrackEvent("SetCurrentView",  
                new Dictionary<string, string>()  
                    {  
                        {"SettingViewTo", currentTab},  
                    }  
                );  

            drawer.CloseDrawer(GravityCompat.Start);  
            return true;  
        }  


//The code that changes ViewPagers in order to change the views  
public void SetCurrentViewpager(PagerAdapterTypesEnum personTypesEnum = PagerAdapterTypesEnum.OddEven)  
        {  
            switch (personTypesEnum)  
            {  
                case PagerAdapterTypesEnum.OddEven:  
                    if (CurrentViewPagerAdapter is PagerAdapterOddEven)  
                        return;  
                    Title = GetString(Resource.String.app_name) + GetString(Resource.String.AppNameDash) +  
                            GetString(Resource.String.Common_OddEven);  
                    CurrentTabLayout.RemoveAllTabs();  
                    CurrentTabLayout.Visibility = ViewStates.Visible;  
                    CurrentTabLayout.AddTab(CurrentTabLayout.NewTab()  
                        .SetText(GetString(Resource.String.OddEven_ResultsTabTitle)));  
                    CurrentTabLayout.AddTab(CurrentTabLayout.NewTab()  
                        .SetText(GetString(Resource.String.OddEven_DelaysTabTitle)));  
                    CurrentTabLayout.AddTab(CurrentTabLayout.NewTab()  
                        .SetText(GetString(Resource.String.OddEven_NotificationsTabTitle)));  
                    CurrentTabLayout.TabGravity = TabLayout.GravityFill;  

                    CurrentViewPagerAdapter = new PagerAdapterOddEven  
                        (SupportFragmentManager, CurrentTabLayout.TabCount);  
                    CurrentViewPager.Adapter = CurrentViewPagerAdapter;  

                    CurrentViewPager.AddOnPageChangeListener(null);  
                    break;  
                case PagerAdapterTypesEnum.Search:  
                    if (CurrentViewPagerAdapter is PagerAdapterSearch)  
                        return;  
                    Title = GetString(Resource.String.app_name) + GetString(Resource.String.AppNameDash) +  
                            GetString(Resource.String.Common_Draws);  
                    CurrentTabLayout.RemoveAllTabs();  
                    CurrentTabLayout.Visibility = ViewStates.Visible;  
                    CurrentTabLayout.AddTab(CurrentTabLayout.NewTab()  
                        .SetText(GetString(Resource.String.Search)));  
                    CurrentTabLayout.AddTab(CurrentTabLayout.NewTab()  
                        .SetText(GetString(Resource.String.NumbersSearch_BonusAndFrequency)));  
                    CurrentTabLayout.AddTab(CurrentTabLayout.NewTab()  
                        .SetText(GetString(Resource.String.NumbersPredictions_Title_WithoutTone)));  
                    CurrentTabLayout.TabGravity = TabLayout.GravityFill;  

                    CurrentViewPagerAdapter = new PagerAdapterSearch  
                        (SupportFragmentManager, CurrentTabLayout.TabCount);  
                    CurrentViewPager.Adapter = CurrentViewPagerAdapter;  

                    CurrentViewPager.AddOnPageChangeListener(  
                        new TabLayout.TabLayoutOnPageChangeListener(CurrentTabLayout));  
                    CurrentTabLayout.TabSelected += (object sender, TabLayout.TabSelectedEventArgs e) =>  
                    {  
                        CurrentViewPager.SetCurrentItem(e.Tab.Position, true);  
                    };  
                    break;  
                case PagerAdapterTypesEnum.Columns:  
                    if (CurrentViewPagerAdapter is PagerAdapterColumns)  
                        return;  
                    Title = GetString(Resource.String.app_name) + GetString(Resource.String.AppNameDash) +  
                            GetString(Resource.String.Common_Columns);  
                    CurrentTabLayout.RemoveAllTabs();  
                    CurrentTabLayout.Visibility = ViewStates.Visible;  
                    CurrentTabLayout.AddTab(CurrentTabLayout.NewTab()  
                        .SetText(GetString(Resource.String.Columns_ColumnsStatistics)));  
                    CurrentTabLayout.AddTab(CurrentTabLayout.NewTab()  
                        .SetText(GetString(Resource.String.Columns_ColumnsResultsPerNumbers)));  
                    CurrentTabLayout.TabGravity = TabLayout.GravityFill;  

                    CurrentViewPagerAdapter = new PagerAdapterColumns  
                        (SupportFragmentManager, CurrentTabLayout.TabCount);  
                    CurrentViewPager.Adapter = CurrentViewPagerAdapter;  

                    CurrentViewPager.AddOnPageChangeListener(  
                        new TabLayout.TabLayoutOnPageChangeListener(CurrentTabLayout));  
                    CurrentTabLayout.TabSelected += (object sender, TabLayout.TabSelectedEventArgs e) =>  
                    {  
                        CurrentViewPager.SetCurrentItem(e.Tab.Position, true);  
                    };  
                    break;  
                case PagerAdapterTypesEnum.Settings:  
                    if (CurrentViewPagerAdapter is PagerAdapterSettings)  
                        return;  
                    Title = GetString(Resource.String.app_name) + GetString(Resource.String.AppNameDash) +  
                            GetString(Resource.String.Common_OptionsTitle);  
                    CurrentTabLayout.RemoveAllTabs();  
                    CurrentTabLayout.Visibility = ViewStates.Gone;  
                    CurrentTabLayout.AddTab(CurrentTabLayout.NewTab()  
                        .SetText(GetString(Resource.String.Columns_ColumnsStatistics)));  

                    CurrentViewPagerAdapter = new PagerAdapterSettings  
                        (SupportFragmentManager, CurrentTabLayout.TabCount);  
                    CurrentViewPager.Adapter = CurrentViewPagerAdapter;  

                    CurrentViewPager.AddOnPageChangeListener(  
                        new TabLayout.TabLayoutOnPageChangeListener(CurrentTabLayout));  
                    CurrentTabLayout.TabSelected += (object sender, TabLayout.TabSelectedEventArgs e) =>  
                    {  
                        CurrentViewPager.SetCurrentItem(e.Tab.Position, true);  
                    };  

                    break;  
                case PagerAdapterTypesEnum.Information:  
                    if (CurrentViewPagerAdapter is PagerAdapterInfo)  
                        return;  
                    Title = GetString(Resource.String.app_name) + GetString(Resource.String.AppNameDash) +  
                            GetString(Resource.String.Common_Info);  
                    CurrentTabLayout.RemoveAllTabs();  
                    CurrentTabLayout.Visibility = ViewStates.Gone;  

                    CurrentViewPagerAdapter = new PagerAdapterInfo  
                        (SupportFragmentManager, 1);  
                    CurrentViewPager.Adapter = CurrentViewPagerAdapter;  

                    break;  
            }  
        }  

Also for reference i will add the code of OnCreateView from the class that produce this crash although i dont think so the problem is there:

public class FragmentDrawsNumbersFrequency : AndroidX.Fragment.App.Fragment  
    {  
...  

public override View OnCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState)  
        {  
            if (inflater == null)  
                inflater = (LayoutInflater)Activity.GetSystemService(Context.LayoutInflaterService);  
            var view = inflater.Inflate(Resource.Layout.FragmentDrawsNumbersFrequency, container, false);  

            TimeToTextView = view.FindViewById<TextView>(Resource.Id.TimeToTextView);  
            TimeFromTextView = view.FindViewById<TextView>(Resource.Id.TimeFromTextView);  
            TimeFromButton = view.FindViewById<Button>(Resource.Id.TimeFromButton);  
            TimeToButton = view.FindViewById<Button>(Resource.Id.TimeToButton);  
            ListNameTextView = view.FindViewById<TextView>(Resource.Id.ListNameTextView);  
            DateToTextView = view.FindViewById<TextView>(Resource.Id.DateToTextView);  
            ShowInstructionsImageButton = view.FindViewById<ImageButton>(Resource.Id.ShowInstructionsImageButton);  
            ShowDateRangeUnlockImageButton = view.FindViewById<ImageButton>(Resource.Id.ShowDateRangeUnlockImageButton);  
            DateFromButton = view.FindViewById<Button>(Resource.Id.DateFromButton);  
            DateToButton = view.FindViewById<Button>(Resource.Id.DateToButton);  
            UnluckySpinner = view.FindViewById<Spinner>(Resource.Id.UnluckySpinner);  
            NumbersFrequencySpinner = view.FindViewById<Spinner>(Resource.Id.NumbersFrequencySpinner);  
            NumbersOrBonusSpinner = view.FindViewById<Spinner>(Resource.Id.NumbersOrBonusSpinner);  
            ClearHistoryButton = view.FindViewById<Button>(Resource.Id.ClearHistoryButton);  
            SearchMostLessViewsButton = view.FindViewById<Button>(Resource.Id.SearchMostLessViewsButton);  
            DailySearchSwitch = view.FindViewById<Switch>(Resource.Id.DailySearchSwitch);  
            SeperateCheckBox = view.FindViewById<CheckBox>(Resource.Id.SeperateCheckBox);  
            ShowListsCheckBox = view.FindViewById<CheckBox>(Resource.Id.ShowListsCheckBox);  

            TimeFromButton.Text = GetString(Resource.String.Common_DefaultFromTimePickerButton);  
            TimeToButton.Text = GetString(Resource.String.Common_DefaultToTimePickerButton);  

            RestoreHistoryList(Settings.DrawsNumbersFrequencyHistoryListViewKey);  
            HistoryListView = view.FindViewById<ListView>(Resource.Id.HistoryListView);  
            HistoryListViewAdapter = new HistoryListViewAdapter(Activity, HistoryList);  
            HistoryListView.Adapter = HistoryListViewAdapter;  
            HistoryListViewAdapter.NotifyDataSetChanged();  

            KinoUrl = GetString(Resource.String.KinoStats_KinoUrl_GR);  
            KinoParams = GetString(Resource.String.KinoStats_KinoParameters_GR);  

            ShowInstructionsImageButton.Click += (sender, e) => { ShowInstructions(); };  
            DailySearchSwitch.Click += (sender, e) =>  
            {  
                DisplayFunctions.SetDailySearch(DailySearchSwitch, DateToButton, DateToTextView,  
                    ShowDateRangeUnlockImageButton, Settings.DailySearchesDrawsNumbersFrequencyKey);  
            };  

            DisplayFunctions.LoadDailySearch(DailySearchSwitch, DateToButton, DateToTextView,  
                ShowDateRangeUnlockImageButton, Settings.DailySearchesDrawsNumbersFrequencyKey);  
            PurchaseFunctions.EnableDateRange(DateToTextView, DateToButton, ShowDateRangeUnlockImageButton);  

            TimeFromButton.Click += (sender, e) => { ShowTimePicker(TimePickerEnum.From, FromDrawNumber, TimeFromButton); };  

            TimeToButton.Click += (sender, e) => { ShowTimePicker(TimePickerEnum.To, ToDrawNumber, TimeToButton); };  

            DateFromButton.Click += (sender, e) => { SelectDateFrom(DateFromButton); };  

            DateToButton.Click += (sender, e) => { SelectDateTo(DateToButton); };  

            ClearHistoryButton.Click += (sender, e) => { PromptClearHistory(); };  

            ShowDateRangeUnlockImageButton.Click += (sender, e) => { UnlockDateRange(); };  

            SearchMostLessViewsButton.Click += async (sender, e) =>  
            {  
                await ShowMostLessViews().ConfigureAwait(false);  
            };  
            FillKinoNums();  
            SetNumbersFrequencySpinner();  

            SetNumbersOrBonusSpinner();  

            //UnluckySpinner  
            SetUnluckySpinner();  
            return view;  
        }  

Am i doing something heavy work here? should i change something or add a delay somewhere?
Thanks

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