EnemyAI NullReferenceException

Hey, i'm trying to make a hack and slash game, based on the tutorials, but, i'm with a problem, that's my code:

using UnityEngine; using System.Collections;

public class EnemyAI : MonoBehaviour { public Transform target; public int moveSpeed; public int rotationSpeed; private Transform myTransform; public GameObject go;

 // void awake(){
 //  myTransform = GetComponent<Transform>();
 //  }
 
 // Use this for initialization
 void Start () {
     myTransform = GetComponent<Transform> ();
     GameObject go = GameObject.FindGameObjectWithTag("Player");
     target = go.transform;
     
 }
 
 void Update () {
     Debug.DrawLine (target.position, myTransform.position); 
     
     // Look at target
     myTransform.rotation = Quaternion.Slerp(myTransform.rotation , Quaternion.LookRotation(target.position - myTransform.position) , rotationSpeed * Time.deltaTime);
     
     // Move towards target
     myTransform.position += myTransform.forward * moveSpeed * Time.deltaTime ;
     
     transform.position = myTransform.position;
     transform.rotation = myTransform.rotation;
 }

}

it says there's a null reference exception on the line number 19 : "target = go.transform;" please help me PS: Sorry my bad English