Removing from a list

I have a list of Transforms of enemies that a healer will heal. However, at times the player will kill and enemy and then I need to remove him from the list. I attempted the following approach:

         if (canHeal && !playerWithinRange ) {
             CheckDistance ();
             if (enemyToHealDistance < distanceCanHeal) {
                 CheckNullList();
                 _healTarget = healGroup[Random.Range (0, healGroup.Count)];
                 goblinAlertEnemyHeal.enemyToHeal = _healTarget.gameObject;                
                 StartCoroutine ("HealTiming");
             }
             else {
                 CheckNullList();
                 _healTarget = healGroup[Random.Range (0, healGroup.Count)];
                 goblinAlertEnemyHeal.enemyToHeal = _healTarget.gameObject;
                 _state = State.GotoEnemyToHeal;
             }
         }
     }
     
     int _listCntr;
     void CheckNullList() {
         _listCntr = 0;
         foreach (Transform bob in healGroup) {
             Debug.Log (bob.name);
             if (bob != null)
                 return;
             else {
                 healGroup.Remove (bob);
                 healGroup.RemoveAt (_listCntr);
             }
         }
     }

I am thinking that when the player kills the enemy and destroys the object, that position in the list should be null. But, it doesn't turn null and I'm getting the error :

"MissingReferenceException: The object of type 'Transform' has been destroyed but you are still trying to access it." at
goblinAlertEnemyHeal.enemyToHeal = _healTarget.gameObject;

I have a few ideas that might take a while to get it working, but I thought this approach would be the easiest.

Does anyone know how I can turn that missing reference into something I can check for and remove from the list?

thanks