Can you please tell me why tab isn't closing?

<Window x:Class="WpfApplication212.Window6"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Title="Window6" Height="300" Width="300" xmlns:igWindows="http://infragistics.com/Windows">
    <Grid>
       
        <Grid.RowDefinitions>
            <RowDefinition/>
            <RowDefinition/>
            <RowDefinition/>
        </Grid.RowDefinitions>
       
        <igWindows:XamTabControl x:Name="tabControl">
            <igWindows:TabItemEx Header="tabItemEx" ExecutingCommand="TabItemEx_ExecutingCommand">
                <Grid />
            </igWindows:TabItemEx>
        </igWindows:XamTabControl>
       
        <Button Content="click" Click="Button_Click" Grid.Row="2"></Button>
       
    </Grid>
</Window>

        private void Button_Click(object sender, RoutedEventArgs e)
        {
            (tabControl.SelectedItem as TabItemEx).ExecuteCommand(TabItemExCommands.Close);
        }