adding children to a node that has List of Nodes

[f1gh]

Hi All:

Okay i am trying to add elements to a List<Node <T>>, i.e. the list is a compilation of generic Nodes. Each node has a <T> data field, has a List<Node <T>> children list, and has Node<T> parent. Here is the code i am trying to write but keep getting NullPointerException:

    public Node<T> addChild(T child) {

        Node toInsert = new Node(child);
       int numchild = getNumberOfChildren();
       System.out.println("This node has: "+numchild);

       //basically if this node doesn't have a parent Node
       if(this.parent ==null){
            //this line of code is where i am getting NullPointerException
            this.children.add(toInsert);
            toInsert.setParentNode(this);
            toInsert.children=null;
       }
       else
       {
        this.children.add(toInsert);

       }

       return this;
    }

somewhere in my main method i am creating the node:
Node<String> s1 = new Node<String>(new String("hello"));
Node<String> s2 = s1.addChild(new String("jack"));

and than ofcourse i further add nodes and childs so that end i almost have a list that branches out like a tree i.e.

           hello
          /      \
        jack    nancy
         |       |
        John    jacob

any help suggestions would be welcomed as i can't figure out the cause of the NullPointer unless the "this" reference is wrong.