Hi, I am following the instruction https://developer.vuforia.com/resources/dev-guide/swappable-databases-unity to dynamically load and destroy datasets.  The load and activate is ok.  Even deactivate is ok.  However, when I try to destroy the dataset,  it generates errors as

Could not destroy trackable with id 1

Could not destroy trackable with id 2

...

This is the testing code I am using 

using UnityEngine;
using System.Collections;
using System.Collections.Generic;

public class qcartest : MonoBehaviour, ITrackerEventHandler
{

    private QCARBehaviour qcarbhv;
    private DataSet dataSet = null;

	// Use this for initialization
	void Start () 
        {
            qcarbhv = FindObjectOfType(typeof(QCARBehaviour)) as QCARBehaviour;
            qcarbhv.RegisterTrackerEventHandler(this);
	}
	
	// Update is called once per frame
	void Update () 
        {
            if (Input.GetKeyDown("l"))
            {
                ImageTracker imageTracker = TrackerManager.Instance.GetTracker(Tracker.Type.IMAGE_TRACKER) as ImageTracker;
                var datasetPath = Application.persistentDataPath + "/SoundItOut2.xml";
                dataSet = imageTracker.CreateDataSet();
                if (!dataSet.Load(datasetPath, DataSet.StorageType.STORAGE_ABSOLUTE))
                {
                    Debug.Log("Failed loading Dataset @ " + datasetPath);
                    return;
                }

                Debug.Log("Dataset @ " + datasetPath + " loaded!");
                imageTracker.ActivateDataSet(dataSet);
            }


            if (Input.GetKeyDown("u"))
            {
                StartCoroutine("Co_Unload");
            }

	}

    public void OnInitialized()
    {
    }

    public void OnTrackablesUpdated()
    {
    }


    IEnumerator Co_Unload()
    {
        ImageTracker imageTracker = TrackerManager.Instance.GetTracker(Tracker.Type.IMAGE_TRACKER) as ImageTracker;
        imageTracker.Stop();
        yield return new WaitForSeconds(0.5f);

        IEnumerable<DataSet> activeDataSets = imageTracker.GetActiveDataSets();
        List<DataSet> activeDataSetsList = new List<DataSet>();

        foreach (var item in activeDataSets)
        {
            activeDataSetsList.Add(item);
        }

        foreach (var item in activeDataSetsList)
        {
            imageTracker.DeactivateDataSet(item);
            Debug.Log("deactivated " + item.Path);
        }

        yield return new WaitForSeconds(0.5f);
        foreach (var item in activeDataSetsList)
        {
            imageTracker.DestroyDataSet(item, true);
            Debug.Log("destroyed " + item.Path);
        }

        yield return new WaitForSeconds(0.5f);
        imageTracker.Start();
    }

}

Please help what I am doing wrong.  Thanks.