cant kill more than one enemy C#

simpy put using the script below i cant shoot after i kill the first enemy because of an issue at lines 42 and 75, both lines say "EnemyAi enemyAi = enemy.GetComponent();" and the exact error is "The object of type 'EnemyAi' has been destroyed but you are still trying to access it." can anybody help with this?

 using UnityEngine;
 using System.Collections;
 using System.Collections.Generic;
 
 public class PistolRay : MonoBehaviour {
 
     bool canFire = true;
     public GameObject decal;
     public float clipAmmo = 8;
     public float storedAmmo = 80;
     float missingShots = 0;
     GameObject player;
     GameObject enemy;
     GameObject characterControler;
     bool reloading = false;
     private List<EnemyAi> enemies = new List<EnemyAi>();
 
     // Use this for initialization
     void Start () {
         player = GameObject.Find("Player");
         enemy = GameObject.Find("EnemyAI");
     }
     
     // Update is called once per frame
     void Update () {
         CharacterControler characterControler = player.GetComponent<CharacterControler>();
         characterControler.ammoCount = clipAmmo;
         characterControler.totalAmmo = storedAmmo;
         var enemyObjects = GameObject.FindGameObjectsWithTag("Enemy");
         foreach(GameObject enemyObj in enemyObjects){
         EnemyAi enemyAi = enemyObj.GetComponent<EnemyAi>();
         if(enemyAi != null){
         enemies.Add(enemyAi);
         }
         }
         RaycastHit hit;
         Ray shooterRay = new Ray(transform.position, transform.forward);
 
         if(Input.GetButton("Fire1") && canFire && clipAmmo > 0){
                 StartCoroutine (SoundCooldown(1));
                 foreach(EnemyAi enemy in enemies){
             EnemyAi enemyAi = enemy.GetComponent<EnemyAi>();
             enemyAi.couldHearGun = true;
             }
         }
         if(Input.GetButton("Fire1") && canFire && clipAmmo > 0){
             Debug.DrawRay (transform.position, transform.forward, Color.green);
                 if(Physics.Raycast(shooterRay, out hit, Mathf.Infinity)){
                 if(hit.collider.tag == "Enemy"){
                     hit.transform.GetComponent<EnemyCC>().health -= 15;
                 }
             }
             var hitRotation = Quaternion.FromToRotation(Vector3.up, hit.normal);
             Instantiate(decal, hit.point, hitRotation);    
             canFire = false;
             clipAmmo -= 1;
             missingShots += 1;
             StartCoroutine(FireRate(1));
             }
         if(Input.GetButtonDown("Fire1") && canFire && clipAmmo == 0 && !reloading){
             reloading = true;
             StartCoroutine(ReloadTime(3));
         }
 
         if(Input.GetButtonDown("Reload") && storedAmmo != 8 && !reloading){
             reloading = true;
             StartCoroutine(ReloadTime(3));
         }
         }
     public IEnumerator FireRate (float delay){
         yield return new WaitForSeconds(delay);
         canFire = true;
     }
     public IEnumerator SoundCooldown(float delay){
         EnemyAi enemyAi = enemy.GetComponent<EnemyAi>();
         yield return new WaitForSeconds(delay);
         enemyAi.couldHearGun = false;
 
     }
     public IEnumerator ReloadTime (float delay){
         yield return new WaitForSeconds(delay);
         reloading = false;
         if(storedAmmo >= 8){
             storedAmmo -= missingShots;
             clipAmmo += missingShots;
             missingShots = 0;
         }
     }
 }