trying to make a tree view with mvc and viewmodel - asp.net-mvc

Question

I try to make a treeview with jsTree plugin.
So In viewmodel I have a function GetTreeViewList- which returns a ViewModel. It should get the first parent node 0 and then it calls GetChildNodes() function - which should take Child Nodes.
But when I try to create the rootNode of type CategoryViewModel() - it gives me an exception that the object is no set to an instance of an object
public class CategoryViewModel
{
public List<CategoryLanguages> lstCategories { get; set; }
private readonly ICategoriesLanguagesRepository catRepository;
public CategoryViewModel() : this(new CategoryLanguageRepository())
{
}
public CategoryViewModel(ICategoriesLanguagesRepository catRepository)
{
this.catRepository = catRepository;
}
public string EmployeeCode { get; set; }
public string EmployeeName { get; set; }
public IList<CategoryViewModel> ChildNodes { get; set; }
public CategoryViewModel GetTreeVeiwList()
{
CategoryLanguages parrentNodes = catRepository.GetAllParrentCaregories();
CategoryViewModel rootNode = new CategoryViewModel()
{
EmployeeCode = parrentNodes.CategoryLanguagesID.ToString(),
EmployeeName = parrentNodes.Title
};
GetChildNode(rootNode, parrentNodes);
return rootNode;
}
private void GetChildNode(CategoryViewModel rootNode,CategoryLanguages item)
{
if (rootNode != null)
{
List<CategoryLanguages> childNodes= catRepository.GetChildNodes(item);
if (childNodes.Count > 0)
{
foreach (var childRootNode in childNodes)
{
CategoryViewModel chdnode = new CategoryViewModel { EmployeeCode = childRootNode.CategoryLanguagesID.ToString(), EmployeeName = childRootNode.Title };
GetChildNode(chdnode, childRootNode);
rootNode.ChildNodes.Add(chdnode);
}
}
}
Here is my Controller
public class CategoryController : Controller
{
public ActionResult Index()
{
CategoryViewModel ob = new CategoryViewModel();
ob.GetTreeVeiwList();
return View(ob);
}
}