Error using Serge Zab's helper for optgroup dropdowns

I'm trying to work with the optgroup dropdown helper from Serge Zab that can be found here.

This is my category table:

As you can see I have a category and a category can also be categoryparent from a category. I want to have the parentcategorys as optgroup and the children as options of the optgroup. (Only the children can be selected)

In my ViewModel:

public short? CategoryId { get; set; }
public IEnumerable<ReUzze.Helpers.GroupedSelectListItem> GroupedTypeOptions { get; set; }

In my Controller:

[Authorize] // USER NEEDS TO BE AUTHORIZED
public ActionResult Create()
{
    ViewBag.DropDownList = ReUzze.Helpers.EnumHelper.SelectListFor<Condition>();

    var model = new ReUzze.Models.EntityViewModel();
    PutTypeDropDownInto(model);
    return View(model);
}

[NonAction]
private void PutTypeDropDownInto(ReUzze.Models.EntityViewModel model)
{
    model.GroupedTypeOptions = this.UnitOfWork.CategoryRepository.Get()
         .OrderBy(t => t.ParentCategory.Name).ThenBy(t => t.Name)
         .Select(t => new GroupedSelectListItem
         {
             GroupKey = t.ParentId.ToString(),
             GroupName = t.ParentCategory.Name,
             Text = t.Name,
             Value = t.Id.ToString()
         }
     );
}

In my View:

@Html.DropDownGroupListFor(m => m.CategoryId, Model.GroupedTypeOptions,  "[Select a type]")    

When I try to run this I always get the error:

Object reference not set to an instance of an object.

I get this error on this rule : .OrderBy(t => t.ParentCategory.Name).ThenBy(t => t.Name)

Can anybody help me find a solution for this problem?