How to view the Selected Header of all Hierarchical levels in SfTreeNavigator

<Window.DataContext>
   <local:ViewModel/>
</Window.DataContext>
<Grid>
   <navigation:SfTreeNavigator ItemsSource="{Binding TreeCollection}" NavigationMode="Extended">
      <navigation:SfTreeNavigator.ItemTemplate>
         <HierarchicalDataTemplate ItemsSource="{Binding SubItems}">
            <TextBlock Text="{Binding Header}"/>
         </HierarchicalDataTemplate>
      </navigation:SfTreeNavigator.ItemTemplate>
   </navigation:SfTreeNavigator>
</Grid>

public class TreeModel : NotificationObject
  {
     private String _header;
     public String Header
      {
        get
        {
          return _header;
        }
        set
        {
          _header = value;
           RaisePropertyChanged("Header");
        }
      }
 
     private ObservableCollection<TreeModel> _subItems;
 
     public ObservableCollection<TreeModel> SubItems
      {
        get
        {
          return _subItems;
        }
        set
        {
          _subItems = value;
        }
      }
     public TreeModel()
      {
         SubItems = new ObservableCollection<TreeModel>();
      }
  }
public class NotificationObject : INotifyPropertyChanged
  {
     public void RaisePropertyChanged(string propName)
      {
 
         if (this.PropertyChanged != null)
               this.PropertyChanged(this, new PropertyChangedEventArgs(propName));
      }
     public event PropertyChangedEventHandler PropertyChanged;
  }
 
public class ViewModel
  {
     private ObservableCollection<TreeModel> _collection;
     public ObservableCollection<TreeModel> TreeCollection
      {
       get
        {
          return _collection;
        }
       set
        {
          _collection = value;
        }
      }
     TreeModel secondChild;
     private void AddChild(TreeModel child)
      {
        for (int i = 1; i <= 15; i++)
          {
            TreeModel firstChild = new TreeModel() { Header = child.Header.ToString()+ i.ToString() };
            child.SubItems.Add(firstChild);
            if(i%3 == 0)
              {
                for(int j = 1; j <= 15; j++)
                 {
                    secondChild = new TreeModel() { Header = firstChild.Header.ToString() + j.ToString() };
                    firstChild.SubItems.Add(secondChild);
                 }
              }
            else
              {
                for(int k = 1; k <= 10; k++)
                 {
                    secondChild = new TreeModel() { Header = firstChild.Header.ToString() + k.ToString() };
                    firstChild.SubItems.Add(secondChild);
                 }
              }
          }
      }
     public ViewModel()
      {
         TreeCollection = new ObservableCollection<TreeModel>();
         TreeModel WPF = new TreeModel() { Header = "WPF" };
         AddChild(WPF);
         TreeModel WinRT = new TreeModel() { Header = "WinRT" };
         AddChild(WinRT);
         TreeModel UWP = new TreeModel() { Header = "UWP" };
         AddChild(UWP);
         TreeCollection.Add(WPF);
         TreeCollection.Add(WinRT);
         TreeCollection.Add(UWP);
       }
  }