Проблема с групировкой данных

Уже несколько дней бьюсь с проблемой группировки данных. По примерам сделать не получается, либо группировка вообще не выводиться ни как, либо пишет что itemsSourse должен быть пуст перед записью. Помогите понять в чем проблема.

<Window x:Class="Buch.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    xmlns:local="clr-namespace:Buch"
    mc:Ignorable="d"
    Title="MainWindow" Height="350" Width="1000">
<Window.Resources>
    <CollectionViewSource x:Key="selectedClient" Source="{Binding SelectedClient.Purchases}">
        <CollectionViewSource.GroupDescriptions>
            <PropertyGroupDescription PropertyName="Date"/>
        </CollectionViewSource.GroupDescriptions>
    </CollectionViewSource>
</Window.Resources>
<Grid x:Name="clientsList">
    <Grid.RowDefinitions>
        <RowDefinition />
    </Grid.RowDefinitions>
    <Grid.ColumnDefinitions>
        <ColumnDefinition Width="*"/>
        <ColumnDefinition Width="*"/>
    </Grid.ColumnDefinitions>
    <Grid Grid.Row="0" Grid.Column="0">
        <Grid.RowDefinitions>
            <RowDefinition Height="50"/>
            <RowDefinition />
        </Grid.RowDefinitions>
        <StackPanel Grid.Row="0" Orientation="Horizontal" HorizontalAlignment="Center" Margin="174,0">
            <Button Content="Добавить" Margin="10" Command="{Binding AddCommand}"/>
            <Button Content="Удалить" Margin="10" Command="{Binding DeleteCommand}"
            CommandParameter="{Binding ElementName=clients, Path=SelectedItem}"/>
        </StackPanel>
        <ListBox Grid.Row="1" x:Name="clients" ItemsSource="{Binding Clients}" SelectedItem="{Binding SelectedClient}">
            <ListBox.ItemTemplate>
                <DataTemplate>
                    <StackPanel>
                        <TextBlock FontSize="24" Text="{Binding Name}"/>
                        <TextBlock FontSize="18" Text="{Binding Telephone}"/>
                        <TextBlock FontSize="18" Text="{Binding Balance}"/>
                        <TextBlock FontSize="18" Text="{Binding Address}"/>
                    </StackPanel>
                </DataTemplate>
            </ListBox.ItemTemplate>
        </ListBox>
    </Grid>
    <ListView Grid.Column="1" ItemsSource="{Binding SelectedClient.Purchases}">
        <ListView.View>
            <GridView>
                <GridViewColumn DisplayMemberBinding="{Binding Date}">Дата</GridViewColumn>
                <GridViewColumn DisplayMemberBinding="{Binding Product.Type}">Тип товара</GridViewColumn>
                <GridViewColumn DisplayMemberBinding="{Binding Product.Material}">Материал</GridViewColumn>
                <GridViewColumn DisplayMemberBinding="{Binding Product.Size}">Размер</GridViewColumn>
                <GridViewColumn DisplayMemberBinding="{Binding Price}">Цена</GridViewColumn>
                <GridViewColumn DisplayMemberBinding="{Binding Count}">Количество</GridViewColumn>
            </GridView>
        </ListView.View>
        <ListView.GroupStyle>
            <GroupStyle>
                <GroupStyle.ContainerStyle>
                    <Style TargetType="{x:Type GroupItem}">
                        <Setter Property="Template">
                            <Setter.Value>
                                <ControlTemplate TargetType="{x:Type GroupItem}">
                                    <Expander IsExpanded="True" 
                                              Background="#FF112255"
                                              BorderBrush="#FF112255" 
                                              Foreground="#FFEEEEEE" 
                                              BorderThickness="1,1,1,5">
                                        <Expander.Header>
                                            <DockPanel>
                                                <TextBlock FontWeight="Bold" 
                                                           Text="{Binding Date}" 
                                                           Margin="5,0,0,0" 
                                                           Width="100"/>
                                            </DockPanel>
                                        </Expander.Header>
                                        <Expander.Content>
                                            <ItemsPresenter/>
                                        </Expander.Content>
                                    </Expander>
                                </ControlTemplate>
                            </Setter.Value>
                        </Setter>
                    </Style>
                </GroupStyle.ContainerStyle>
            </GroupStyle>
        </ListView.GroupStyle>
    </ListView>
</Grid>

 public class Client : INotifyPropertyChanged
{
    public int Id { get; set; }
    private string name;
    private string telephone;
    private int balance;
    private string address;
    public ICollection<Purchase> Purchases { get; set; }
    public string Address
    {
        get { return address; }
        set
        {
            address = value;
            OnPropertyChanged("Address");
        }
    }
    public string Name
    {
        get { return name; }
        set
        {
            name = value;
            OnPropertyChanged("Name");
        }
    }
    public string Telephone
    {
        get { return telephone; }
        set
        {
            telephone = value;
            OnPropertyChanged("Telephone");
        }
    }
    public int Balance
    {
        get { return balance; }
        set
        {
            balance = value;
            OnPropertyChanged("Balance");
        }
    }

    public event PropertyChangedEventHandler PropertyChanged;
    public void OnPropertyChanged([CallerMemberName]string prop = "")
    {
        PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(prop));
    }
}
public class ClientVM : INotifyPropertyChanged
{
    ApplicationContext db;
    RelayCommand addCommand;
    RelayCommand deleteCommand;
    IEnumerable<Client> clients;
    private Client selectedClient;
    public Client SelectedClient
    {
        get { return selectedClient; }
        set { selectedClient = value;
            OnPropertyChanged("SelectedClient");
        }
    }

    public IEnumerable<Client> Clients
    {
        get { return clients; }
        set
        {
            clients = value;
            OnPropertyChanged("Clients");
        }
    }
    public ClientVM()
    {
        db = new ApplicationContext();
        db.Clients.Load();
        db.Purchases.Load();
        db.Products.Load();
        Clients = db.Clients.Local.ToBindingList();
    }
    public RelayCommand AddCommand
    {
        get
        {
            return addCommand ??
                (addCommand = new RelayCommand((o) =>
                {
                    ClientWindow clientWindow = new ClientWindow(new Client());
                    if (clientWindow.ShowDialog() == true)
                    {
                        Client client = clientWindow.Client;
                        db.Clients.Add(client);
                        db.SaveChanges();
                    }
                }));
        }
    }
    public RelayCommand DeleteCommand
    {
        get
        {
            return deleteCommand ??
                (deleteCommand = new RelayCommand((selecterItem) =>
                {
                    if (selecterItem == null) return;
                    Client client = selecterItem as Client;
                    db.Clients.Remove(client);
                    db.SaveChanges();
                }));
        }
    }
    public event PropertyChangedEventHandler PropertyChanged;
    public void OnPropertyChanged([CallerMemberName]string prop = "")
    {
        PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(prop));
    }
}