Instantiate GameObject Parent targetting issue.

I'm working on a script that targets an enemy, and when said enemy is targeted it Instantiate a GameObject that will display health and such.

Instantiating and spawning it ON a parent object works fine, however it won't spawn as a child (and therefore won't travel with the target as it moves.) I've got other scripts that spawn in the same fashion, so I'm not sure on the issue.

Also getting an error of NullReferenceException: Object reference not set to an instance of an object issue.

 private void TargetEnemy() {
         if(selectedTarget == null) {
             SortTargetsByDistance();
             selectedTarget = targets[0];
         }
         else {
             int index = targets.IndexOf(selectedTarget);
             
             if(index < targets.Count - 1) {
                 index++;
             }
             else {
                 index = 0;
             }
             DeselectTarget();
             selectedTarget = targets[index];
         }
         SelectTarget();
     }
     private void SelectTarget() {
      GameObject targetHUD = Instantiate(TargetHUD, selectedTarget.position, selectedTarget.rotation) as GameObject;
     targetHUD.transform.parent = selectedTarget;      
 
     }
     private void DeselectTarget() {
         Destroy (GameObject.FindWithTag("TargetHUD"));
         selectedTarget = null;
         
     }