Instantiate object through a coroutine problem

I have this script that spawn the npc in random waypoints position within a limit, in which the following conditions are checked:

• These waypoints must be out of the camera's field of view
  

• Waypoints must be far from the player's position
  

• The maximum npc to be spawned is 150

  ---

Each of these waypoints contains the attached "Waypoints" script. When spawning these npc, it takes the "Waypoints" component as the currentWaypoint.waypoint. The strange thing is that while I had a simple scene the code worked perfectly without any error, but in the current situation, with a complex scene (with several buildings and irregular terrain), some of these nps keep the currentWaypoint.waypoint null

Here's my code:

 void Start()
 {
     StartCoroutine(Spawn());
 }
 
 IEnumerator Spawn()
 {
     int count = 0;

     while (count < pedestriansToSpawn)
     {
         Transform child = transform.GetChild(Random.Range(0, transform.childCount - 1));

         if (CanSpawn(child) && child.GetComponent<GetDistance>().distance > 50)
         {
             GameObject obj = Instantiate(PedestrianSelected());
             obj.transform.parent = parent;
             obj.name = string.Format("Pedestrian {0}", count + 1);
             obj.GetComponent<WaypointNavigator>().currentWaypoint.waypoint = child.GetComponent<Waypoints>();
             obj.GetComponent<NavMeshAgent>().Warp(new Vector3(child.position.x, child.position.y + 1, child.position.z));
         }
         else
             count--;

         yield return new WaitForSeconds(.25f);
         count++;
     }
 }