Drop drag wpf перетаскивание вкладок ошибка

Делаю перетаскивание вкладок,как в браузере.Выдает ошибку Операция недопустима, когда ItemsSource используется. Вместо этого получите доступ и измените элементы с помощью ItemsControl.ItemsSource в строке int sourceIndex = TabItem.Items.IndexOf(tabItemSource); Подскажите,что я пропустил. вот код с#

 private TabItem GetTargetTabItem(object originalSource)
    {
        var current = originalSource as DependencyObject;
        while (current != null)
        {
            var tabItem = current as TabItem;
            if (tabItem != null)
            {
                return tabItem;
            }
            current = VisualTreeHelper.GetParent(current);
        }
        return null;
    }
    private void TabItem_PreviewMouseMove(object sender, MouseEventArgs e)
    {
        var tabItem = e.Source as TabItem;
        if (tabItem == null)
            return;
        if (Mouse.PrimaryDevice.LeftButton == MouseButtonState.Pressed)
        {
            DragDrop.DoDragDrop(tabItem, tabItem, DragDropEffects.All);
        }
    }

    private void TabItem_Drop(object sender, DragEventArgs e)
    {
        var tabItemTarget = GetTargetTabItem(e.OriginalSource);
        if (tabItemTarget != null)
        {
            var tabItemSource = (TabItem)e.Data.GetData(typeof(TabItem));
            if (tabItemTarget != tabItemSource)
            {
                int sourceIndex = TabItem.Items.IndexOf(tabItemSource);
    int targetIndex = TabItem.Items.IndexOf(tabItemTarget);
                TabItem.Items.Remove(tabItemSource);
                TabItem.Items.Insert(targetIndex, tabItemSource);
                TabItem.Items.Remove(tabItemTarget);
                TabItem.Items.Insert(sourceIndex, tabItemTarget);
                TabItem.SelectedIndex = targetIndex;
            }

и xml

<Window x:Class="WPFDynamicTab.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Title="Dynamic Tab" Height="300" Width="527" WindowStartupLocation="CenterScreen">
<TabControl>
    <TabControl Name="TabItem" ItemsSource="{Binding}" SelectionChanged="tabDynamic_SelectionChanged">
        <TabControl.Resources>
                <Style TargetType="TabItem" >
                <Setter Property="AllowDrop" Value="True"/>
                <EventSetter Event="PreviewMouseMove" Handler="TabItem_PreviewMouseMove"/>
                <EventSetter Event="Drop" Handler="TabItem_Drop"/>
            </Style>
            <DataTemplate x:Key="TabHeader" DataType="TabItem">
                <DockPanel>
                    <Button Name="btnDelete" DockPanel.Dock="Right" Margin="5,0,0,0" Padding="0" Click="btnDelete_Click" CommandParameter="{Binding RelativeSource={RelativeSource AncestorType={x:Type TabItem}}, Path=Name}">
                        <!--<Image Source="/delete.gif" Height="11" Width="11"></Image>-->
                    </Button>
                    <TextBlock Text="{Binding RelativeSource={RelativeSource AncestorType=TabItem }, Path=Header}" />
                </DockPanel>
            </DataTemplate>
        </TabControl.Resources>
    </TabControl>
</TabControl>