I have a problem with the healthbar in a network multiplayer

Hi there. I have this "A short summary of the script"

HEALTHBAR:

 public const int maxHealth = 100;
 public const int maxMana = 100;
 public bool destroyOnDeath;

 [SyncVar(hook = "OnChangeHealth")]
 public int currentHealth = maxHealth;
 public int currentMana = maxMana;

 public RectTransform healthBar;
 public RectTransform manaBar;



 void Start ()
 {
     
     healthBar =(RectTransform) Hud.HealthBar;
     manaBar = Hud.ManaBar;
 }

 public void TakeDamage(int amount)
 {
     if (!isServer)
         return;

     currentHealth -= amount;
     if (currentHealth <= 0)
     {
         if (destroyOnDeath)
         {
             Destroy(gameObject);
         } 
         else
         {
             currentHealth = maxHealth;

             // called on the Server, invoked on the Clients
             RpcRespawn();
         }
     }
 }

 void OnChangeHealth (int currentHealth )
 {
     healthBar.sizeDelta = new Vector2(currentHealth , healthBar.sizeDelta.y);
 }

}

HUD

 public Transform LifeBar;
 public static RectTransform HealthBar;
 public static RectTransform ManaBar;

 void Start () {

     LifeBar = this.gameObject.transform.FindChild("LifeBar");

     HealthBar = LifeBar.transform.Find("HealthBar").GetComponent<RectTransform>();
     ManaBar = LifeBar.transform.Find ("ManaBar").GetComponent<RectTransform>();
      
 }

BALA

public int Damage; // Use this for initialization void Start () { if (Damage == 0) { Damage = 10; } }

 // Update is called once per frame
 void Update () {

 }

 void OnTriggerEnter(Collider other){

     var hit = other.gameObject;
     var health = hit.GetComponent<Health> ();
     if ((other.gameObject.name != "BulletMulti")) {
         Destroy (this.gameObject);
     }
     if (health != null) {
         health.TakeDamage (Damage);
     }
     //recuerda poner el && (other.gameObject.tag != "Player")

 }

And it gives me an error.

"NullReferenceException: Object reference not set to an instance of an object Health.OnChangeHealth (Int32 currentHealth) (at Assets/Scripts/Health.cs:57) Health.set_NetworkcurrentHealth (Int32 value) Health.TakeDamage (Int32 amount) (at Assets/Scripts/Health.cs:38) Bala.OnTriggerEnter (UnityEngine.Collider other) (at Assets/Scripts/Bala.cs:27)"