Pathfindingscript NullReference error

Hey guys, this script should move an object to one transform after the other and then start at the start waypoint again after going through all Transforms in the array. However everytime the object reaches the last waypoint, a NullReference error occurs because targetWayPoint was not set to an instance of an object. I don't know how to solve this. Could someone help?

Script:

 public float speed;
 public int currentWayPoint;
 public Transform[] Waypoints;
 public bool FixedRotation;
 public bool Loop = true;

 private Quaternion FixedRotate;

 Transform targetWayPoint;

 void Start ()
 {
     this.transform.rotation = FixedRotate;
 }

 void Update ()
 {
     if (targetWayPoint == null)
         targetWayPoint = Waypoints [currentWayPoint];
     
     if (currentWayPoint < this.Waypoints.Length) 
     {
         Walk ();
     }

     if (FixedRotation) 
     {
         transform.rotation = FixedRotate;
     }
 }

 void Walk ()
 {
     if (this.transform.position == targetWayPoint.position) 
     {
         if (currentWayPoint == (Waypoints.Length - 1) && Loop) 
         {
             targetWayPoint = Waypoints [0];
         } 

         else if (currentWayPoint == (Waypoints.Length - 1) && !Loop) 
         {
             GetComponent<WaypointPathFinding> ().enabled = false;
         } 

         else 
         {
             currentWayPoint++;
             targetWayPoint = Waypoints [currentWayPoint];
         }
     }         

     transform.forward = Vector3.RotateTowards (transform.forward, targetWayPoint.position - transform.position, speed * Time.deltaTime, 0.0f);
     transform.position = Vector3.MoveTowards (transform.position, targetWayPoint.position, speed * Time.deltaTime);
 }