Help with my PopUp Damage Text!

Hello, I have a problem with my script to display a text indicating the damage that the player made an enemy , but I checked in and not be wrong . When testing the script I get the error " NullReferenceException : Object reference not set to an instance of an object" . The scripts are as follows :

Health: This script is attached to the player and enemies

 public int HP = 5;
     public int HPMaximo = 5;
     public GameObject PrefabTextoDamagePopUp;
     public List<GameObject> TextoDamagePopUpLista = new List<GameObject>();
     int indiceTextoDamagePopUpListaActual = 0;
 
     public void doDamage(int Damage){
         HP -= Damage;
         TextoDamagePopUpLista[indiceTextoDamagePopUpListaActual] = (GameObject)Instantiate
             (PrefabTextoDamagePopUp, 
                 new Vector3
                 (transform.position.x, transform.position.y + 2, transform.position.z),
                 Quaternion.identity);
         indiceTextoDamagePopUpListaActual++;
         TextoDamagePopUpLista[indiceTextoDamagePopUpListaActual].GetComponent<TextDamagePopUp>().DamageMostrado = Damage;
     }

Arma: This script is attached to the weapon of the player

 public Sprite[] SpriteArray;
     public int[] DamageArray;
     public bool ArmaActiva = false;
     public static int ID;
     public static int DamageActual = 2;
 
 
 
     void OnTriggerEnter2D(Collider2D coll){
         if(ArmaActiva && coll.GetComponent<Health>() && coll.tag != "J"){
             coll.GetComponent<Health>().doDamage(DamageActual);
         }
     }
 }

TextDamagePopUp: This script is attached to the 3d text prefab , which is going to show the damage that the player has made

 public float tiempoVida;
     float tiempoVidaA;
     public int DamageMostrado;
     void Start(){
         tiempoVidaA = tiempoVida;
     }
 
     void FixedUpdate(){
         tiempoVida -= Time.fixedDeltaTime;
         GetComponent<TextMesh>().text = System.String.Empty + DamageMostrado;
 
         if(tiempoVida <= 0)
             Destroy(gameObject);
     }
 
 }

If there is another way to show the damage done in a text, they will thank you if I explain it please! alt text