ObjectTracker NullReferenceException vuforia Dataset

I’m trying to add Dataset dynamically for image tracking , here below my code. When i run unity3D i getting this error.

`using UnityEngine;
using System.Collections;
using Vuforia;
public class CreateData : MonoBehaviour
{

void Start ()
{
VuforiaARController.Instance.RegisterVuforiaStartedCallback (LoadData);
}

DataSet myDataSet;
public void LoadData ()
{
string dataPath = “File:///” + Application.streamingAssetsPath + “/QCAR” + “/100.xml”;
ObjectTracker objectTracker = (ObjectTracker)TrackerManager.Instance.GetTracker ();
Debug.Log (objectTracker);
myDataSet = objectTracker.CreateDataSet ();
objectTracker.ActivateDataSet (myDataSet);
}
}`

Any ideas?