DevExpress CollectionView Collapse Group problem in xamarin forms app

Question

I'm new at xamarin forms and developing an application. I make a popup page to make a menu. I'm using devexpress collectionview in it. My menu is grouped and I have 5 groups. I want collapse that groups but It doesn't work well.It works when page open first time but 2nd time it give me the error: "object reference not set to an instance of an object"

I check the dev express documentation for xamarin forms. There is nothing about grouping but in maui part I could find some information.

https://docs.devexpress.com/MAUI/404126/collection-view/filter-sort-and-group-data#group-collection-view-items

OnAppearing Method :

 protected override void OnAppearing()
    {
            if (dxCollection.ItemCount >= 0)
            {
            dxCollection.CollapseGroup(-1);
            dxCollection.CollapseGroup(-2);
            dxCollection.CollapseGroup(-3);
            dxCollection.CollapseGroup(-4);
            dxCollection.CollapseGroup(-5);
        }
base.OnAppearing();
}

Visual that I want.

Error : It appears page opened 2nd time. First time is working well. What could happen that lost object and give me error. I can not find.

What could I do for debuging ? I can not find anything to controll "null" here.

DxCollectionview Group:

                <!--Group items.-->
            <dxcv:DXCollectionView.GroupDescription >
                <dxcv:GroupDescription FieldName="ANAGRUPADI" GroupInterval="Value"/>
            </dxcv:DXCollectionView.GroupDescription>
            

            <!--Define the group header template.-->
            <dxcv:DXCollectionView.GroupHeaderTemplate  >
                <DataTemplate>
                    <StackLayout Margin="3"  >
                        <Frame CornerRadius="8" Padding="0" Margin="0">
                            <StackLayout  HeightRequest="30" HorizontalOptions="FillAndExpand" Orientation="Horizontal"  BackgroundColor="CadetBlue" >
                                <Label FontFamily="Roboto" TextColor="Black" FontSize="Small" VerticalTextAlignment="Center" HorizontalTextAlignment="Center" HorizontalOptions="CenterAndExpand" Text="{Binding Value}"/>
                                
                                <Image Source="@drawable/popupmenu.png"  HorizontalOptions="End"  HeightRequest="20" />
                                
                                
                            </StackLayout>
                            
                            
                            
                        </Frame>
                            

                        
                       
                    </StackLayout>
                </DataTemplate>
            </dxcv:DXCollectionView.GroupHeaderTemplate>

LayoutChildren method :

 protected override void LayoutChildren(double x, double y, double width, double height)
    {
        dxCollection.CollapseAllGroups();

        base.LayoutChildren(x, y, width, height);
    }

I want to add Xamarin.Kotlin.StdLib but it gives error. I cant find how to add those references. Error :

Version conflict detected for DevExpress.XamarinForms.Core. Install/reference DevExpress.XamarinForms.Core 22.1.8-pre-23026 directly to project İdaServis.Android to resolve this issue.

NU1202 Package Microsoft.Owin 4.2.2 is not compatible with monoandroid13.0 (MonoAndroid,Version=v13.0). Package Microsoft.Owin 4.2.2 supports: net45 (.NETFramework,Version=v4.5)

Answer 1

Firstly, I backup my project. I work on backup project.

My aim is adding Xamarin.Kotlin.StdLib because DevExpress Collectionview works great with this nuget package.

Then open android.csproj file with a text editor. I used VsCode. Add those lines :

<PackageReference Include="Xamarin.Google.Android.Material">
  <Version>1.6.0</Version>
</PackageReference>
<PackageReference Include="Xamarin.AndroidX.Core">
  <Version>1.7.0</Version>
</PackageReference>

After adding these I could add Xamarin.Kotlin.StdLib. But this was not enough. I fill my collectionview from my web service. Sometimes it doesn't collapse groups. Adding a delay time layoutchildren method make it great.

protected override async void LayoutChildren(double x, double y, double width, double height)
    {
        await Task.Delay(250);
        dxCollection.CollapseAllGroups();

        base.LayoutChildren(x, y, width, height);
    }