NullReferenceException: Object reference not set to an instance of an object C#

NullReferenceException: Object reference not set to an instance of an object SaludEnemigo1.OnTriggerEnter2D (UnityEngine.Collider2D collider) (at Assets/Scripts/SaludEnemigo1.cs:62)

This is my entire error, I don't know what to do. Anybody can help me, please?

Those are my codes.

 using UnityEngine;
 using System.Collections;
 
 public class ScoreScript : MonoBehaviour {
 
 
     // Interfaz Grafica 
 
     public GUIText scoreText;
     private int score;
 
 
     void Start () {
         score = 0;
         UpdateScore ();
     }
     
     public void AddScore (int newScoreValue)
         
     {
         score += newScoreValue;
         UpdateScore ();
     }
     
     
     
     void UpdateScore ()
     {
         scoreText.text = "Score: " + score;
     }
 
 }






 using UnityEngine;
 using System.Collections;
 
 /// Comportamiento de la salud de los objetos
 
 public class SaludEnemigo1 : MonoBehaviour {
     
     
     /// Numero de puntos de salud
     
     public int ps = 2;
     
     private GameObject Enemigo1;
     private ScoreScript scoreScript;
     public int ScoreValue;
     
     
     /// Es jugador o enemigo?
     
     public bool esEnemigo = true;
 
     // LLamamos el script score y el tag enemigo1
     
     void start ()
     {
         GameObject scoreScriptObject = GameObject.FindGameObjectWithTag ("ScoreScript");
         if (scoreScriptObject != null) {
             scoreScript = scoreScriptObject.GetComponent <ScoreScript> ();
         }
         if (scoreScript == null) {
             Debug.Log ("No se puede encontrar ScoreScript");
         }
 
         Enemigo1 = GameObject.FindWithTag ("Enemigo1");
         if (Enemigo1 == null) {
                         Debug.Log ("No se encuentra a enemigo1");
                 }
     }
 
 
     
     void OnTriggerEnter2D(Collider2D collider)
     {
         //Es un disparo?
         Disparar disparo = collider.gameObject.GetComponent<Disparar>();
         if (disparo != null)
         {
             // Revisamos si es enemigo o compañero
             if (disparo.esDisparoEnemigo != esEnemigo)
             {
                 ps -= disparo.daño;
                 
                 // Destruimos el disparo
                 // No colocar solo Destroy()
                 //sino eliminara el Script
                 Destroy(disparo.gameObject);
                 
                 if (ps <= 0)
                 {
                     // Muerto + EfectoEspecial de particulas, += score
 
                     scoreScript.AddScore(ScoreValue);
                     EfectosEspecialesScript.Instancia.Explosion(transform.position);
                     EfectosDeSonido.Instancia.ReproducirSonidoExplosion();
                     Destroy(gameObject); 
                     
                 }
                 
             }
         }
     }
     void OnCollisionEnter2D(Collision2D coll)
     {
         if (coll.gameObject.tag == "Player")
             ps = 0; 
         EfectosEspecialesScript.Instancia.ExplosionAsteroide(transform.position);
         Destroy(Enemigo1); 
     }
 }

I want the line "scoreScript.AddScore(ScoreValue);" to work but I don't know what to do, I've searched for 3 h and nothing. Maybe the error it provide in console is for the method I call the script? If it's that, how can I do the best way for instance?

Thanks a lot for your help!