Missing Reference Exception

Hi All,

I've written a Spaceship Adventure game where enemies shoot a spaceship as the spaceship travels through the scene, one of my problems is when a bullet hits the enemy I destroy the gameObject and get the following error:

MissingReferenceException: The object of type 'AudioSource' has been destroyed but you are still trying to access it. Your script should either check if it is null or you should not destroy the object. UnityEngine.AudioSource.Play ()

The way I have it setup is, my defense enemies have the script below attach to them. My understanding is if I destroy the object after a bullet hits it, I would assume this script wouldn't be called again? is that correct? because if this gets called again I would understand why I'm getting a "MissingReferenceException". Please help :)

 using UnityEngine;
 using System.Collections;
 
 public class DefenseEnemy : MonoBehaviour {
     
     private float lastUpdate;
     private int[] shootingPositions = { 180, 205, 144 };
     private float rotateDefenseTime = 1.0f;
     private float whenToUpdateInSeconds = 2.0f;
     private float bulletLife = 10.0f;
     
     public GameObject bullet;
     public float bulletSpeed = 1000.0f; 
     private float explotionLength = 1.0f;
     
     //Enemy Health
     private float health;
     
     private bool dead = false;
     
     public GameObject explotionPrefab;
     //Audio
     public AudioSource crashSound,shootSound;
     
     void Start(){
         health = 2.0f;
     }
     void Update () {
         if(health == 0 || dead)
             return;
         //Every time change the Y rotation of the defense enemy
         if(Time.time - lastUpdate >= whenToUpdateInSeconds && !dead){
             //Shoot Bullet 
             iTween.ShakePosition(gameObject,new Vector3(0.4f,0.3f,0.4f),0.5f);
             lastUpdate = Time.time;
             iTween.RotateTo(gameObject,new Vector3(270,shootingPositions[Random.Range(0,3)],0),rotateDefenseTime);
             ShootBullet();
           }
     }
     
     void OnCollisionEnter(Collision col){
         if(col.gameObject.tag == "SpaceshipBullet" && health > 0){
             health--;
             if(health == 0){
                 iTween.ShakePosition(gameObject,new Vector3(0.4f,0.3f,0.4f),0.5f);
                 crashSound.Play();
                 GameObject explotion = (GameObject)
                     Instantiate(explotionPrefab,gameObject.transform.position, Quaternion.identity);
                 dead = true;
                 Destroy(explotion,explotionLength);
                 Destroy(gameObject,explotionLength);
             }
         }
     }
     void ShootBullet(){
         shootSound.Play();
         GameObject newBullet = (GameObject)Instantiate(bullet,bullet.transform.position,bullet.transform.rotation);
         newBullet.transform.localScale = new Vector3(0.12f,0.12f,0.12f);
         newBullet.rigidbody.AddRelativeForce(Vector3.back * bulletSpeed);
         Destroy(newBullet,bulletLife);
     }
 }