GUIText update error

GUITextUpdate.js

private var numPoints= 0;

function Start()
{
    UpdateDisplay();
}

function UpdateDisplay()
{
    guiText.text = "Points: "+numPoints;
}

function AddPoints()
{
    numPoints++;
    UpdateDisplay();
}

DetectCollision.js

var gu: GUITextUpdate;

  function OnCollisionEnter(theCollision : Collision){

 if(theCollision.gameObject.name == "Wicket"){

      Debug.Log("Hit the cube");

      gu.AddPoints();

 }

}

giving this error

NullReferenceException: Object reference not set to an instance of an object
DetectCollission.OnCollisionEnter (UnityEngine.Collision theCollision) (at Assets/Script/DetectCollission.js:6)

You need to assign gu at some point, the inspector is probably the best way for you.