Got a Error about reffering a object i can not fix

So i have this Basic AI for a mob to follow the player in 2D but it wont work. Can somebody help me with this? This is the Error:

NullReferenceException: Object reference not set to an instance of an object EnemyAI.AI () (at Assets/Scripts/EnemyAI.cs:24) EnemyAI.Update () (at Assets/Scripts/EnemyAI.cs:19)

Code: using UnityEngine; using System.Collections;

 public class EnemyAI : MonoBehaviour 
 {
     Transform Hero;
     public float AISpeed = 5f;
     public float MaxDistance = 10f;
     public float Mindistance = 2f;
 
     void Start ()
     {
         Hero = GameObject.FindGameObjectWithTag("player").transform;  
     }
 
     void Update ()
     {
         transform.LookAt (Hero);
         AI ();
     }
 
     void AI()
     {
         if(Vector3.Distance (transform.position, Hero.position) >= Mindistance)
         {
             transform.position += transform.forward*AISpeed*Time.deltaTime;
 
             if(Vector3.Distance (transform.position, Hero.position) >= MaxDistance)
             {
 
             }
         }
     }
     
 }