Change UIPropertyMetadata
public static readonly DependencyProperty SelectedItemProperty =
DependencyProperty.RegisterAttached("SelectedItem", typeof(object), typeof(TreeViewHelper), new UIPropertyMetadata(null, SelectedItemChanged));
to FrameworkPropertyMetadata
public static readonly DependencyProperty SelectedItemProperty =
DependencyProperty.RegisterAttached("SelectedItem", typeof(object), typeof(TreeViewHelper), new FrameworkPropertyMetaData(null, FrameworkPropertyMetadataOptions.BindsTwoWayByDefault, SelectedItemChanged));