Accessing Events Throwing Null Exception

Hello I have a script accessing an instantiated game object of Vuforia from Hierarchy and adding a component to it:

IEnumerable<TrackableBehaviour> tbs = TrackerManager.Instance.GetStateManager().GetTrackableBehaviours();

foreach (TrackableBehaviour tb in tbs)
{
if (tb.TrackableName == "Fashion")
{

if (tb.name == "New Game Object")
{

// change generic name to include trackable name
tb.gameObject.name = ++counter + "DynamicImageTarget-" + tb.TrackableName;

// add additional script components for trackable
tb.gameObject.AddComponent<DefaultTrackableEventHandler>();
tb.gameObject.AddComponent<TurnOffBehaviour>();

Up to here it is working but at this point/line when i try to add

tb.gameObject.GetComponent<DefaultTrackableEventHandler>().OnTargetFound.AddListener(DressFound);

is throwing null exception in runtime

When I try

tb.gameObject.GetComponent<DefaultTrackableEventHandler>().OnTargetFound.RemoveAllListeners();

throwing null exception in runtime as well

However when i try

tb.gameObject.GetComponent<DefaultTrackableEventHandler>().enable = false;

it is working and disabling script component

void DressFound()
{
augmentation.SetActive(true);

}

Why i can't reach events on the script?

 

I solved the issue with changing code in DefaultTrackableEventHandler script



from



public UnityEvent OnTargetFound;

public UnityEvent OnTargetLost;



to



public UnityEvent OnTargetFound = new UnityEvent();

public UnityEvent OnTargetLost = new UnityEvent();