/// <summary>
/// Handles the CollectionChanged event of the Things control.
/// </summary>
/// <param name="sender">The source of the event.</param>
/// <param name="e">The <see cref="System.Collections.Specialized.NotifyCollectionChangedEventArgs"/> instance containing the event data.</param>
void ThingsCollectionChanged(object sender, System.Collections.Specialized.NotifyCollectionChangedEventArgs e)
{
RemoveFirstItem.ChangeCanExecute();
AddItem.ChangeCanExecute();
}