CSharp Script Error NullReferenceException

This is the error : NullReferenceException: Object reference not set to an instance of an object EnemyAttack.Attack () (at Assets/SCRIPT/EnemyAttack.cs:44) EnemyAttack.Update () (at Assets/SCRIPT/EnemyAttack.cs:34)

This is my code : using UnityEngine; using System.Collections;

 public class EnemyAttack : MonoBehaviour {
 
     public float maxDistance;
     public float coolDownTimer;
     public PlayerHealth ph;
 
     private Transform myTransform;
     public Transform target;
 
 
 
 
     // Use this for initialization
     void Start () {
 
 
         GameObject go = GameObject.FindGameObjectWithTag ("Player");
         target = go.transform;
         myTransform = go.transform;
         maxDistance = 3;
         coolDownTimer = 0;
 
         ph = (PlayerHealth)go.GetComponent (typeof(PlayerHealth));
     
     }
     
     // Update is called once per frame
     void Update () {
         float distance = Vector3.Distance (target.position, myTransform.position);
         if (distance < maxDistance) {
             Attack ();
 
         }
 
         if (coolDownTimer > 0) {
             coolDownTimer = coolDownTimer * Time.deltaTime;
         }
         if (coolDownTimer < 0) {
             coolDownTimer = 0;
         }
     }
     void Attack () {
         if (coolDownTimer == 0) {
             ph.ChangeHealth (-10);
             coolDownTimer = 1;
         }
     }
 }