How to expand selected TreeViewItemAdv on Button Click in WPF TreeViewAdv?

<syncfusion:TreeViewAdv ItemsSource="{Binding TreeItems}"
                        x:Name="mainTreeview">
    <syncfusion:TreeViewAdv.ItemContainerStyle>
        <Style TargetType="{x:Type syncfusion:TreeViewItemAdv}">
            <Setter Property="IsExpanded"
                    Value="{Binding IsExpanded, UpdateSourceTrigger=PropertyChanged}" />
        </Style>
    </syncfusion:TreeViewAdv.ItemContainerStyle>

    <syncfusion:TreeViewAdv.ItemTemplate>
        <HierarchicalDataTemplate ItemsSource="{Binding SubItems}">
            <StackPanel Orientation="Horizontal">
                <ContentPresenter Content="{Binding Header}" />
            </StackPanel>
        </HierarchicalDataTemplate>
    </syncfusion:TreeViewAdv.ItemTemplate>
</syncfusion:TreeViewAdv>

<Button Content="Expand Selected Item"
        Width="200"
        Height="30"
        Click="Button_Click"
        Grid.Column="1"
        VerticalAlignment="Center" />

// --- MainWindow.xaml.cs ---
public partial class MainWindow : Window
{
    public MainWindow()
    {
        InitializeComponent();
        this.DataContext = new ViewModel();
    }

    private void Button_Click(object sender, RoutedEventArgs e)
    {
        if (mainTreeview.SelectedItem != null)
        {
            if (mainTreeview.SelectedItem is TreeViewItemAdv)
            {
                (mainTreeview.SelectedItem as TreeViewItemAdv).IsExpanded = true;
            }
            else
            {
                if (mainTreeview.SelectedItem is Model)
                {
                    (mainTreeview.SelectedItem as Model).IsExpanded = true;
                }
            }
        }
        else
        {
            MessageBox.Show("Before click Button select any one TreeViewitem");
        }
    }
}

// --- ViewModel.cs ---
public class ViewModel
{
    public ViewModel()
    {
        TreeItems = new ObservableCollection<Model>();
        PopulateData();
    }

    public ObservableCollection<Model> TreeItems { get; set; }

    //Root Node
    private void PopulateData()
    {
        for (int i = 0; i < 6; i++)
        {
            Model Root1 = new Model() { Header = "Root" + i };
            PopulateSubItems(Root1);
            TreeItems.Add(Root1);
        }
    }

    //Sub Node
    private void PopulateSubItems(Model Root)
    {
        for (int i = 1; i <= 1; i++)
        {
            Model SubItem1 = new Model() { Header = Root.Header + " Item" + i };
            Root.SubItems.Add(SubItem1);
        }
    }
}

// --- Model.cs ---
public class Model : INotifyPropertyChanged
{
    public Model()
    {
        SubItems = new ObservableCollection<Model>();
    }

    private string _header;
    public string Header
    {
        get { return _header; }
        set
        {
            _header = value;
            raisepropertychanged("Header");
        }
    }

    private bool _isExpanded;
    public bool IsExpanded
    {
        get { return _isExpanded; }
        set
        {
            _isExpanded = value;
            raisepropertychanged("IsExpanded");
        }
    }

    public ObservableCollection<Model> SubItems { get; set; }

    #region INotifyPropertyChanged Members
    public event PropertyChangedEventHandler PropertyChanged;

    public void raisepropertychanged(string name)
    {
        if (PropertyChanged != null)
        {
            PropertyChanged(this, new PropertyChangedEventArgs(name));
        }
    }
    #endregion
}