When I try updating in Xamarin Forms I get a System.NullReferenceException

When I try updating in Xamarin Forms I get an System.NullReferenceException HResult=0x80004003 Message=Object reference not set to an instance of an object.

Here is the code.

namespace QiXApp.Services.Data

{

public class SxCaseDataService : BaseService, ISxCaseDataService
{
    private readonly IGenericRepository _genericRepository;



    public SxCaseDataService(IGenericRepository genericRepository,
        IBlobCache cache = null) : base(cache)
    {
        _genericRepository = genericRepository;
    }



    public async Task<IEnumerable<SxCase>> GetAllSxCasesAsync()
    {
        List<SxCase> sxCasesFromCache =
            await GetFromCache<List<SxCase>>(CacheNameConstants.AllSxCases);

        if (sxCasesFromCache != null)//loaded from cache
        {
            return sxCasesFromCache;
        }
        else
        {
            UriBuilder builder = new UriBuilder(ApiConstants.BaseApiUrl)
            {
                Path = ApiConstants.SxCaseEndpoint
            };

            var sxCases = await _genericRepository.GetAsync<List<SxCase>>(builder.ToString());

            await Cache.InsertObject(CacheNameConstants.AllSxCases, sxCases, DateTimeOffset.Now.AddSeconds(20));

            return sxCases;
        }
    }



    public async Task<SxCase> UpdateSxCase(SxCase selectedSxCase)
    {


        UriBuilder builder = new UriBuilder(ApiConstants.BaseApiUrl)
        {
            Path = ApiConstants.SxCaseEndpoint
        };

        var result = await _genericRepository.PatchAsync(builder.ToString(), selectedSxCase);

        return result;
    }




}

}

namespace QiXApp.Contracts.Services.Data

{ public interface ISxCaseDataService { Task> GetAllSxCasesAsync();

    Task<SxCase> UpdateSxCase(SxCase selectedSxCase);



}

}

namespace QiXApp.ViewModels

{ public class SxCaseDetailViewModel : ViewModelBase { private SxCase _selectedSxCase; private readonly ISxCaseDataService _sxCaseDataService;

    public SxCaseDetailViewModel(IConnectionService connectionService,
        INavigationService navigationService, IDialogService dialogService)
        : base(connectionService, navigationService, dialogService)
    { }

    public ICommand UpdateSxCaseCommand => new Command(OnUpdateSxCase);
    public ICommand ReadDescriptionCommand => new Command(OnReadDescription);

    public SxCase SelectedSxCase
    {
        get => _selectedSxCase;
        set
        {
            _selectedSxCase = value;
            OnPropertyChanged();
        }
    }

    public override async Task InitializeAsync(object sxCase)
    {
        SelectedSxCase = (SxCase)sxCase;
    }

    private async void OnUpdateSxCase()
    {
       await _sxCaseDataService.UpdateSxCase(SelectedSxCase);

        MessagingCenter.Send(this, MessagingConstants.UpdateSxCase, SelectedSxCase);
        await _dialogService.ShowDialog("Case details updated.", "Success", "OK");
    }

    private void OnReadDescription()
    {
        DependencyService.Get<ITextToSpeech>().ReadText(SelectedSxCase.Facesheet);
    }
}

}

<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
         xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
         xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
         xmlns:d="http://xamarin.com/schemas/2014/forms/design"
         xmlns:utility="clr-namespace:QiXApp.Utility;assembly=QiXApp"
         xmlns:dataform="clr-namespace:Syncfusion.XForms.DataForm;assembly=Syncfusion.SfDataForm.XForms"
         utility:ViewModelLocator.AutoWireViewModel="True"
         mc:Ignorable="d"
         x:Class="QiXApp.Views.SxCaseDetailView">
<ContentPage.Content>
    <StackLayout>
        <dataform:SfDataForm x:Name="dataForm" DataObject="{Binding SelectedSxCase}"  
                             AutoGenerateItems="false"
                             NotifyPropertyChanges="True">

            <dataform:SfDataForm.Items>
                <dataform:DataFormTextItem Name="SxCaseId" Editor="Text"/>
                <dataform:DataFormDateItem Name="Dos" Editor="Date"/>
                <dataform:DataFormTextItem Name="Facesheet" Editor="Text"/>
                <dataform:DataFormTextItem Name="Record" Editor="Text"/>
                <dataform:DataFormSegmentItem Name="Complication" Editor="Switch"/>
                <dataform:DataFormSegmentItem Name="ReturnOr" Editor="Switch"/>


            </dataform:SfDataForm.Items>
        </dataform:SfDataForm>
        <Button Text="Update Case" Style="{StaticResource RegularButton}" Command="{Binding UpdateSxCaseCommand}" Grid.Row="5" Grid.Column="0" Grid.ColumnSpan="2" HorizontalOptions="Center"></Button>

    </StackLayout>

</ContentPage.Content>