Multi Select problem about xamarin.forms.datagrid HOT 2 CLOSED

<ContentPage.Content>
        <StackLayout>
            <Label Text="Welcome to Xamarin.Forms!"
                VerticalOptions="CenterAndExpand" 
                HorizontalOptions="CenterAndExpand" />
            <dg:DataGrid FontSize="12" ItemsSource="{Binding Path=_cashBoxOperations}" SelectionEnabled="True" SelectedItem="{Binding Path=SelectedOperation}"
               RowHeight="80" HeaderHeight="50" BorderColor="#CCCCCC" HeaderBackground="#E0E6F8"
               PullToRefreshCommand="{Binding RefreshCommand}" IsRefreshing="{Binding IsRefreshing}">
                <dg:DataGrid.HeaderFontSize>
                    <OnIdiom  x:TypeArguments="x:Double">
                        <OnIdiom.Tablet>15</OnIdiom.Tablet>
                        <OnIdiom.Phone>13</OnIdiom.Phone>
                    </OnIdiom>
                </dg:DataGrid.HeaderFontSize>
                <dg:DataGrid.Columns>
                    <dg:DataGridColumn Title="Дата" PropertyName="Date" Width="2*"/>
                    <dg:DataGridColumn Title="Взято" PropertyName="Taken" Width="2*"/>
                    <dg:DataGridColumn Title="Тип" PropertyName="Type" Width="2*"/>
                    <dg:DataGridColumn Title="До" PropertyName="LessIn" Width="2*"/>
                    <dg:DataGridColumn Title="После" PropertyName="Less" Width="2*"/>
                </dg:DataGrid.Columns>
                <dg:DataGrid.RowsBackgroundColorPalette>
                    <dg:PaletteCollection>
                        <Color>#F2F2F2</Color>
                        <Color>#FFFFFF</Color>
                    </dg:PaletteCollection>
                </dg:DataGrid.RowsBackgroundColorPalette> 
                <dg:DataGrid.Resources>
                    <ResourceDictionary>
                        <conv:StreakToColorConverter x:Key="StreakToColorConverter"/>
                    </ResourceDictionary>
                </dg:DataGrid.Resources>
            </dg:DataGrid>
        </StackLayout>

    </ContentPage.Content>
</ContentPage>

 public class SecondPageVM : BaseViewModel
    {
        private CashBoxOperations _SelectedOperation;
        public CashBoxOperations SelectedOperation
        {
            get => _SelectedOperation;
            set
            {
                SetProperty(ref _SelectedOperation, value);
                Application.Current.MainPage.DisplayAlert("Error",$"{SelectedOperation.Id + " " + SelectedOperation.Date}","Ok");
            }
        }
        private ObservableCollection<CashBoxOperations> _cashBoxList;
        public ObservableCollection<CashBoxOperations> _cashBoxOperations
        {
            get => _cashBoxList;
            set => SetProperty(ref _cashBoxList, value);
        }
        public Controller _controller;
        private Salons _selectedSalon;
        public Salons SelectedSalon
        {
            get => _selectedSalon;
            set => SetProperty(ref _selectedSalon, value);
        }
        public SecondPageVM(Salons _sel)
        {
            SelectedSalon = _sel;
            _controller = new Controller();
            _cashBoxOperations = _controller.GetOperations(SelectedSalon);
        }
    }