Can't get simultaneous VuMarks working

Hello, I'm using the following script to display a different model depending on the encoded data of the VuMark.  I have basically followed this tutorial https://youtu.be/NDtRvgC-cfw 

Here is the code: 

### csharp

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

public class VuMarkTrackerBehavior : MonoBehaviour, ITrackableEventHandler {     public GameObject[] models;

    TrackableBehaviour vuMark_TrackableBehavior;

    VuMarkManager vuMark_Manager;

    void Start()     {         foreach(var model in models)         {             model.gameObject.SetActive(false);         }

        vuMark_TrackableBehavior = GetComponent<TrackableBehaviour>();                 if (vuMark_TrackableBehavior != null)         {             vuMark_TrackableBehavior.RegisterTrackableEventHandler(this);         }

        vuMark_Manager = TrackerManager.Instance.GetStateManager().GetVuMarkManager();

    }

    public void OnTrackableStateChanged(TrackableBehaviour.Status previousStatus, TrackableBehaviour.Status newStatus)     {         Debug.Log("Trackable " + vuMark_TrackableBehavior.name +                   " " + vuMark_TrackableBehavior.CurrentStatus +                   " -- " + vuMark_TrackableBehavior.CurrentStatusInfo);

        if (newStatus == TrackableBehaviour.Status.DETECTED ||             newStatus == TrackableBehaviour.Status.TRACKED ||             newStatus == TrackableBehaviour.Status.EXTENDED_TRACKED)         {             OnTrackerFound();         }         else if(previousStatus == TrackableBehaviour.Status.TRACKED && newStatus == TrackableBehaviour.Status.NO_POSE)         {             OnTrackerLost();         }     }

    void OnTrackerFound()     {         foreach (var item in vuMark_Manager.GetActiveBehaviours())         {             int id = System.Convert.ToInt32(item.VuMarkTarget.InstanceId.NumericValue);             models[id].gameObject.SetActive(true);         }     }

    void OnTrackerLost()     {         foreach (var item in vuMark_Manager.GetActiveBehaviours())         {             int id = System.Convert.ToInt32(item.VuMarkTarget.InstanceId.NumericValue);             models[id].gameObject.SetActive(false);         }     } }

###

Next I set "Max Simultaneous Tracked Images" to 4 and then hit "Play"

The result is that when holding up 1 VuMark I see the model as expected.  However, if I then hold up and 2nd VuMark I get the following error:

NullReferenceException: Object reference not set to an instance of an object VuMarkTrackerBehavior.OnTrackerFound () (at Assets/AA/Scripts/VuMarkTrackerBehavior.cs:68) VuMarkTrackerBehavior.OnTrackableStateChanged (Vuforia.TrackableBehaviour+Status previousStatus, Vuforia.TrackableBehaviour+Status newStatus) (at Assets/AA/Scripts/VuMarkTrackerBehavior.cs:57) Vuforia.TrackableBehaviour.RegisterTrackableEventHandler (Vuforia.ITrackableEventHandler trackableEventHandler) (at <21bfe61d2ac241f6a9131b858645aad9>:0) VuMarkTrackerBehavior.Start () (at Assets/AA/Scripts/VuMarkTrackerBehavior.cs:25)

This refers to the line "foreach (var item in vuMark_Manager.GetActiveBehaviours())"

Additionally, in the scene hierarchy a Clone of the VuMark object is instantiated.  Looks like VuMark(Clone).

Please advise on what's needed to track multiple VuMarks so that I can use the encoded data to determine which model to display for that VuMark.

Thank you.