How to expand selected TreeViewItemAdv on Button Click?

<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" Click="Button_Click" Height="30" Grid.Column="1" VerticalAlignment="Center"/>

Class: MainWindow.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");
            }
        }
}
Class: ViewModel
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);
            }
        }
    }
Class: Model
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
    }