Attack player script not working?

i am making a zombie game, but i keep getting an error about an object not set, and wont add the damage to the player. her is the error:

NullReferenceException: Object reference not set to an instance of an object ZombieAttack.Update () (at Assets/Scripts/Zombie/ZombieAttack.cs:22)

but it is assigned and working like it should, exept for the health going down.

here is the code attached to the player:

 using UnityEngine;
 using System.Collections;
 
 public class ZombieAttack : MonoBehaviour
 {
     public GameObject[] enemies;
     LifeScript life;
     void Start()
     {
         life = GetComponent<LifeScript>();
         enemies = GameObject.FindGameObjectsWithTag("zombie");
     }
     
     void Update()
     {
         enemies = GameObject.FindGameObjectsWithTag("zombie");
         for (int i = 0; i < enemies.Length; i++)
         {
             Debug.Log(i);
             if (enemies[i].GetComponent<ZombieWalking>().canAttack == true)
             {
                 life.health -= enemies[i].GetComponent<ZombieWalking>().dam;
                 enemies[i].GetComponent<ZombieWalking>().canAttack = false;
             }
             
         }
     }
 }

and here is the code the script is referencing on the zombie:

 using UnityEngine;
 using System.Collections;
 
 public class ZombieWalking : MonoBehaviour {
 
     public float speed;
     public GameObject player;
     public float distanceToPlayer;
     public float cooldown;
     public bool canAttack = false;
     public int dam = 6;
     // Use this for initialization
     void Start () {
         player = GameObject.FindGameObjectWithTag ("rick");
     }
     
     // Update is called once per frame
     void Update () {
         distanceToPlayer = Vector3.Distance (transform.position, player.transform.position);
         if (distanceToPlayer > 1.6f && distanceToPlayer < 10.0f) {
             transform.position = Vector3.MoveTowards (transform.position, player.transform.position, speed);
             transform.LookAt(player.transform.position);
         }
         if (distanceToPlayer < 2)
         {
             if (!canAttack)
             {
                 cooldown -= Time.deltaTime;
                 if (cooldown < 0.1f)
                 {
                     canAttack = true;
                     cooldown = 3;
                 }
             }
         }
         if (distanceToPlayer > 2)
         {
             canAttack = false;
             cooldown = 3;
         }
     }
 }

please help if you can :(