Object reference not set to an instance of an object #5

 MyClass myobj = null;
 int hash = myobj.GetHashCode();

  SomeFunc(MyClass myparam)
  {
       if (myparam == null)
           throw new ArgumentNullException("myparam must be a MyClass object");
  }

  SomeFunc(MyClass myparam)
  {
       if (myparam == null)
           throw new ArgumentNullException("myparam", typeof(MyClass));
  }

  public class MyArgumentNullException : ArgumentNullException
  {
      public MyArgumentNullException(string name, Type type) 
          :base(name + "must be a " + type.Name + " object");
  }

private XElement FillSubCatagories(XElement cat) {
        IEnumerable<XElement> list = cat.Descendants();
        int catTypeID = c.Attribute("CatTypeID") == null ? 1 : Int32.Parse(cat.Attribute("CatTypeID").Value);
        foreach (XElement c in list) {
        if(!String.IsNullOrEmpty(cat.Attribute("CatID").Value))
        {
            int parentID = Int32.Parse(cat.Attribute("CatID").Value);
            XElement sub = new XElement("sub",
                from s in db.Categories
                where s.CatTypeID.Equals(catTypeID) && s.ParentID.Equals(parentID)
                select new XElement("Category",
                     s.CatID == null ? null : new XAttribute("CatID", s.CatID),
                     s.ParentID == null ? null : new XAttribute("ParentID", s.ParentID),
                     s.CatTitle == null ? null : new XAttribute("CatTitle", s.CatTitle),
                     s.CatTypeID == null ? null : new XAttribute("CatTypeID", s.CatTypeID),
                     s.shortDesc == null ? null : new XAttribute("shortDesc", s.shortDesc),
                     s.longDesc == null ? null : new XAttribute("longDesc", s.longDesc),
                     s.CatImage == null ? null : new XAttribute("CatImage", s.CatImage)));
            c.Add(sub);
            FillSubCatagories(c);
            }
        }
        return cat;
    }

int totalValue = 0;
int total = 0;
totalValue = int.Parse("2".ToString()) * int.Parse("2".ToString());
string s = "Yes";
totalValue += s.Equals("Yes",StringComparison.CurrentCultureIgnoreCase) ? 80 : 0;

public static IEnumerable<T> Traverse<T>(IEnumerable<T> source, Func<T, IEnumerable<T>> childSelector)
{
    var queue = new Queue<T>(source);
    while (queue.Any())
    {
        var item = queue.Dequeue();
        yield return item;
        foreach (var child in childSelector(item))
        {
            queue.Enqueue(child);
        }
    }
}

public static IEnumerable<T> Traverse<T>(T root, Func<T, IEnumerable<T>> childSelector)
{
    return Traverse(new[] { root }, childSelector);
}

Category root = new Category();
var searchResult = Traverse(root, item => item.Categories)
            .Where(category => category.Name == "testValue")
            .FirstOrDefault();

Traverse(root, item => item.Categories ?? Enumerable.Empty<Category>())