Can't use any variable from different class/ NullReferenceExcept

Hi everyone! I'm using Vuforia engine with Unity to develop AR apps. I am newbie so got following problems. When I'm trying to get variable from different class in my sources folder. Everything goes well until the target variable is triggered to be change. I have a custom hud which is GUITexture. I want to make it enabled or disabled depending on another class' (Which is ImageTargetTrackabelEventHandler) isBeingTracked boolean variable. During runtime i get error like this:

NullReferenceException: Object reference not set to an instance of an object HudBehaviour.Update () (at Assets/Scripts/HudBehaviour.cs)

My code is following: HudBehaviour Class:

 Public class HudBehaviour : MonoBehaviour {
 
     public GameObject transfer;
 
     private GUITexture hud;
     private ImageTargetTrackableEventHandler checker;
 
     void Awake(){
         hud = GetComponent<GUITexture> ();
         checker = transfer.GetComponent<ImageTargetTrackableEventHandler> ();
         }
 
 
     // Use this for initialization
     void Start () 
     {
 
     }
     
     // Update is called once per frame
     void Update () {
         if (checker.isBeingTracked==true) 
         {
             hud.guiTexture.enabled=true;
         }
         else if (Input.GetKeyDown(KeyCode.A)) {
             hud.enabled=true;
                 }
         //else {
         //    hud.enabled=false;
         //        }
     
     }
 }

Second, ImageTargetTrackableEventHandler Class is like following:

 public class ImageTargetTrackableEventHandler : MonoBehaviour,
                                             ITrackableEventHandler
 {
     #region PUBLIC_MEMBER_VARIABLES
     public bool isBeingTracked=false;
     #endregion PUBLIC_MEMBER_VARIABLES
     
     #region PRIVATE_MEMBER_VARIABLES
     private TrackableBehaviour mTrackableBehaviour;
     #endregion // PRIVATE_MEMBER_VARIABLES
 
     #region PUBLIC_METODS
     void Start()
     {
         mTrackableBehaviour = GetComponent<TrackableBehaviour>();
         if (mTrackableBehaviour)
         {
             mTrackableBehaviour.RegisterTrackableEventHandler(this);
         }
     }
     
 
     /// <summary>
     /// Implementation of the ITrackableEventHandler function called when the
     /// tracking state changes.
     /// </summary>
     public void OnTrackableStateChanged(
                                     TrackableBehaviour.Status previousStatus,
                                     TrackableBehaviour.Status newStatus)
     {
         if (newStatus == TrackableBehaviour.Status.DETECTED ||
             newStatus == TrackableBehaviour.Status.TRACKED ||
             newStatus == TrackableBehaviour.Status.EXTENDED_TRACKED)
         {
             OnTrackingFound();
         }
         else
         {
             OnTrackingLost();
         }
     }
 
     #endregion // PUBLIC_METHODS
 
 
 
     #region PRIVATE_METHODS
     private void OnTrackingFound()
     {
         isBeingTracked = true;
         Renderer[] rendererComponents = GetComponentsInChildren<Renderer>(true);
         Collider[] colliderComponents = GetComponentsInChildren<Collider>(true);
         
         // Enable rendering:
         foreach (Renderer component in rendererComponents)
         {
             component.enabled = true;
         }
         
         // Enable colliders:
         foreach (Collider component in colliderComponents)
         {
             component.enabled = true;
         }
         
         Debug.Log("Trackable " + mTrackableBehaviour.TrackableName + " found");
     }
 
 
     private void OnTrackingLost()
     {
         isBeingTracked = false;
         Renderer[] rendererComponents = GetComponentsInChildren<Renderer>(true);
         Collider[] colliderComponents = GetComponentsInChildren<Collider>(true);
 
         // Disable rendering:
         foreach (Renderer component in rendererComponents)
         {
             component.enabled = false;
         }
 
         // Disable colliders:
         foreach (Collider component in colliderComponents)
         {
             component.enabled = false;
         }
 
         Debug.Log("Trackable " + mTrackableBehaviour.TrackableName + " lost");
     }
 
     #endregion // PRIVATE_METHODS
 }

Please respond anyone who familiar with such problem. Thanks!