NullReferenceException: Object reference not set to an instance of an object SimpleCloudHandler+c__Iterator4.MoveNext ()

I'm doing vuforia + unity app. I have 3D model in server url then parsing the url loading to app. I'm using AssetBundle and www format to display the 3D model from url.

 // Here we handle a cloud target recognition event
 public void OnNewSearchResult(TargetFinder.TargetSearchResult targetSearchResult) {
     // do something with the target metadata
     mTargetMetadata = targetSearchResult.MetaData;
          
         JSONObject json = JSONObject.Parse(mTargetMetadata);
         
         string type=json.GetString("type");
          url=json.GetString("overlay3d");
         
         
     // stop the target finder (i.e. stop scanning the cloud)
     mCloudRecoBehaviour.CloudRecoEnabled = false;
         
         mShowGUIButton = true;
     
         
           GameObject newImageTarget = Instantiate(ImageTargetTemplate.gameObject) as GameObject;
          
         GameObject augmentation = null;
  
         if( augmentation != null )
             augmentation.transform.parent = newImageTarget.transform;
  
         // enable the new result with the same ImageTargetBehaviour:
       //  ImageTargetBehaviour imageTargetBehaviour = mImageTracker.TargetFinder.EnableTracking(targetSearchResult, newImageTarget);
          
 //StartCoroutine(GetAssetBundle());
  
         Debug.Log("Metadata value is " + url );
         StartCoroutine(GetAssetBundle());
         
         
         
         // Build augmentation based on target
 if (ImageTargetTemplate) {
     // enable the new result with the same ImageTargetBehaviour:
     ImageTracker tracker = TrackerManager.Instance.GetTracker<ImageTracker>();
     ImageTargetBehaviour imageTargetBehaviour = 
         (ImageTargetBehaviour)tracker.TargetFinder.EnableTracking(
             targetSearchResult, ImageTargetTemplate.gameObject);
 }
         
         /*
      // str=modelName;
  using (WWW www = new WWW(url)) {
   
      AssetBundle bundle = www.assetBundle;
  g=Instantiate(bundle.mainAsset,new Vector3(0,0,0),Quaternion.identity) as GameObject;
   g.transform.localScale=new Vector3(0.136500f,0.136500f,0.136500f);
 } 
           */
 
          
          GUI.Label(new Rect(10, 10, 300, 70), "Path: " + url);
         
          if (GUI.Button(new Rect(100,300,200,50), "Path: " + url)) {
             // Restart TargetFinder
             mCloudRecoBehaviour.CloudRecoEnabled = true;
         }
         
 }
     
     
   IEnumerator GetAssetBundle ()
 {
  
 www = new WWW(url);
 yield return www;
 Debug.Log("The last BundleURL " + BundleURL );
  
 AssetBundleRequest request = www.assetBundle.LoadAsync("modelipad", typeof(GameObject));
 yield return request;
  
 Instantiate(request.asset, new Vector3(0f, 0f, 0f), Quaternion.identity);
  
 //Put the stream model under the image target
 GameObject ImageTargetUsed = GameObject.Find("ImageTarget");
          GameObject ModelIpadClone = GameObject.Find("modelipad(Clone)");
          ModelIpadClone.transform.parent = ImageTargetUsed.transform;
  
 }