Error: NullReferenceException: Object reference not set to an instance of an object

The basis of the below script is to get my NPC to move from waypoint to waypoint. It is such a simple script but I don't understand why I am receiving the error. Every object is referenced in my hierarchy. :

NullReferenceException: Object reference not set to an instance of an object Basic_AI.Update () (at Assets/Assets/Scripts/Basic_AI.cs:65)

Here are the scripts

 using UnityEngine;
 using System.Collections;
 
 public class Basic_AI : MonoBehaviour {
 
     // This shows the waypoints
     public GameObject[] Waypoint = new GameObject [5];
 
     // This adds a character controller, so the NPC can move through the waypoints.
     private CharacterController controller;
 
     //These are AI Conditionss for the different type of units
     private bool attackAI;
     private bool aiEngage;
     private bool flightAI;
 
     private int unitType;
     private bool agression;
 
     // This is a counter to determine which waypoints have been acocmplished
 
     private int waypointReached = 0;
 
     // This determines how fast the NPC will turn and move towards a waypoint
     private int rotationSpeed = 10;
     private int speed;
 
 
     void Start () {
 
     // This pulls the stistics from the sister script for the NPC.
     bool infantryUnit = GetComponent <Stats_Script>().infantryUnit;
     bool groundUnit = GetComponent <Stats_Script>().groundUnit;
     bool airUnit = GetComponent <Stats_Script>() .airUnit;
      speed = GetComponent <Stats_Script>().speed;
 
     // These are conditions set for each NPC.s
     if (airUnit == true)
         {
             unitType = 3;
             flightAI = true;
         }
 
     else if (groundUnit == true)
         {
             unitType = 2;
             flightAI = false;
             aiEngage = true;
         }
 
     else if (infantryUnit == true)
         {
             unitType = 1;
             flightAI = false;
             aiEngage = true;
         }
     }
     
 
     void Update () {
 
 // ---------------------------------- MOVE TO WAYPOINT --------------------------------------------------
         if (aiEngage == true)
         {
             if (gameObject.transform.position != Waypoint[waypointReached].transform.position)
             {
                 gameObject.transform.rotation = Quaternion.Slerp (gameObject.transform.rotation, Quaternion.LookRotation (Waypoint[waypointReached].transform.position - gameObject.transform.position), rotationSpeed * Time.deltaTime);
                 controller.SimpleMove (transform.forward * speed);
             }
 // ------------------------- WAYPOINT REACHED CONDITIONS ------------------------------------------------
             else if (gameObject.transform.position == Waypoint[waypointReached].transform.position)
             {
                 waypointReached ++;
         }
         }
     }
 }

and here is the referenced Stat Script

 using UnityEngine;
 using System.Collections;
 
 public class Stats_Script : MonoBehaviour {
 
     public bool infantryUnit;
     public bool groundUnit;
     public bool airUnit;
 
     public bool aggression;
 
     public int life;
     public int attack;
     public int speed;
     public int defense;
 
 
     // Use this for initialization
     void Start () {
     
     }
     
     // Update is called once per frame
     void Update () {
         if (infantryUnit == true)
         {
             groundUnit = false;
             airUnit = false;
         }
 
         if (groundUnit == true)
         {
             infantryUnit= false;
             airUnit= false;
         }
 
         if (airUnit == true)
         {
             infantryUnit = false;
             groundUnit = false;
         }
     }
 }