Error in my C# script, Help!

So I have a 2D platformer game, and my enemy has a health bar over its head and what I'm trying to do is make the health bar's scale change to be proportional to the enemy's health. The enemy has 2 health by the way. But I get these errors:

NullReferenceException: Object reference not set to an instance of an object Zombie.UpdateHealthBar () (at Assets/Scripts/Zombie.cs:158) Zombie.Hurt () (at Assets/Scripts/Zombie.cs:116) Bullet.OnTriggerEnter2D (UnityEngine.Collider2D col) (at Assets/Scripts/Bullet.cs:38)

and...

NullReferenceException: Object reference not set to an instance of an object Zombie.Death () (at Assets/Scripts/Zombie.cs:149) Zombie.FixedUpdate () (at Assets/Scripts/Zombie.cs:94)

So, when the "Bullet" hits the "Zombie" its supposed to lose 1 "HP" and then change the "Z_HealthBar" to be proportionate to the enemy's health. This code works fine for my player by the way, so I don't know why I get errors for this.

Enemy's Script: public class Zombie : MonoBehaviour {

     public Transform target;
 
     public int moveSpeed = 2;
     public int rotationSpeed;
     public int maxDistance;
 
     public float jumpPower = 300;
     private bool hasJumped = false;
 
     public int HP = 2;                    // How many times the enemy can be hit before it dies.
     private bool dead = false;            // Whether or not the enemy is dead.
 
     [HideInInspector]
     public bool facingRight = true;
 
     public Transform myTransform;
     private SpriteRenderer spriteRenderX;
     private Animator anim;                      // Reference to the Animator on the player
 
     private SpriteRenderer z_HealthBar;           // Reference to the sprite renderer of the health bar.
     private Vector3 healthScale;                // The local scale of the health bar initially (with full health).
 
     private Bullet bulletHit;
 
     void Awake()
     {
         myTransform = transform;
 
         spriteRenderX = GetComponent<SpriteRenderer>();
 
         // Setting up the references.
         bulletHit = GetComponent<Bullet>();
     }
 
     // Use this for initialization
     void Start()
     {
         GameObject go = GameObject.FindGameObjectWithTag("Player");
 
         target = go.transform;
 
         maxDistance = 5;
     }
 
     void OnCollisionEnter2D(Collision2D coll)
     {
         if (GetComponent<Rigidbody2D>().velocity.y == 0 && coll.gameObject.tag == ("Z_Surface"))
         {
             hasJumped = true;
         }
     }
 
     void FixedUpdate()
     {
         // Set the enemy's velocity to moveSpeed in the x direction.
         //GetComponent<Rigidbody2D>().velocity = new Vector2(transform.localScale.x * moveSpeed, GetComponent<Rigidbody2D>().velocity.y);
 
         if (Vector3.Distance(target.position, myTransform.position) < maxDistance)
         {
             if (target.position.x < myTransform.position.x) myTransform.position -= myTransform.right * moveSpeed * Time.deltaTime; // player is left of enemy, move left
             {
                 if (target.position.x < myTransform.position.x && facingRight)
                     FlipL();
                 
             }
             if (target.position.x > myTransform.position.x) myTransform.position += myTransform.right * moveSpeed * Time.deltaTime; // player is right of enemy, move right
             {
                 if (target.position.x > myTransform.position.x && !facingRight)
                     FlipR();
                 
             }
 
         }
         if (hasJumped)
         {
             GetComponent<Rigidbody2D>().AddForce(transform.up * jumpPower);
             hasJumped = false;
         }
         // If the enemy has zero or fewer hit points and isn't dead yet...
         if (HP <= 0 && !dead)
             // ... call the death function.
             Death();
     }
 
     //Flips sprite
     void FlipL()
     {
         facingRight = !facingRight;
 
         spriteRenderX.flipX = true;
     }
     void FlipR()
     {
         facingRight = !facingRight;
 
         spriteRenderX.flipX = false;
     }
 
     public void Hurt()
     {
         // Reduce the number of hit points by one.
         HP--;
         // Update what the health bar looks like.
         UpdateHealthBar();
     }
 
     void Death()
     {
         // Find all of the sprite renderers on this object and it's children.
         SpriteRenderer[] otherRenderers = GetComponentsInChildren<SpriteRenderer>();
 
         // Disable all of them sprite renderers.
         foreach (SpriteRenderer s in otherRenderers)
         {
             s.enabled = false;
         }
         
         // Set dead to true.
         dead = true;
 
         // Find all of the colliders on the gameobject and set them all to be triggers.
         Collider2D[] cols = GetComponents<Collider2D>();
         foreach (Collider2D c in cols)
         {
             c.isTrigger = true;
         }
         // Move all sprite parts of the enemy to the front
         SpriteRenderer[] spr = GetComponentsInChildren<SpriteRenderer>();
         foreach (SpriteRenderer s in spr)
         {
             s.sortingLayerName = "UI";
         }
 
         // ... disable Zombie script
         GetComponent<Zombie>().enabled = false;
         // ... disable Z Health Bar script
         GetComponent<Z_HealthBarFollow>().enabled = false;
 
         // Play a random audioclip from the deathClips array.
         /*int i = Random.Range(0, deathClips.Length);
         AudioSource.PlayClipAtPoint(deathClips[i], transform.position);*/
     }
     void UpdateHealthBar()
     {
         // Set the scale of the health bar to be proportional to the enemy's health.
         z_HealthBar.transform.localScale = new Vector3(healthScale.x * HP * 0.01f, 1, 1);
     }
 }

Bullet Script: public class Bullet : MonoBehaviour {

     public GameObject hit;        // Prefab of hit effect.
     private Zombie bulletHit;
 
     void Awake()
     {
         // Setting up the references.
         bulletHit = GetComponent<Zombie>();
     }
 
     void Start()
     {
         // Destroy the bullet after 1 seconds if it doesn't get destroyed before then.
         Destroy(gameObject, 1);
     }
 
 
     void OnHit()
     {
         // Create a quaternion with a random rotation in the z-axis.
         Quaternion randomRotation = Quaternion.Euler(0f, 0f, Random.Range(0f, 360f));
 
         // Instantiate the hit where the bullet is with the random rotation.
         Instantiate(hit, transform.position, randomRotation);
 
     }
 
     void OnTriggerEnter2D(Collider2D col)
     {
         // If it hits an enemy...
         if (col.tag == "Enemy")
         {
             // ... find the Enemy script and call the Hurt function.
             col.gameObject.GetComponent<Zombie>().Hurt();
 
             // Call the hit instantiation.
             OnHit();
 
             // Destroy the bullet.
             Destroy(gameObject);
         }
         if (col.tag == "Surface")
         {
             // Call the hit instantiation.
             OnHit();
 
             // Destroy the bullet.
             Destroy(gameObject);
         }
     }
 }