Add to generic list on start problem

I'm doing tests with generic list so dont care about "Pokemon scripts" and don't bother me about copyright and etc, that just for testing purposes..

So, i want that when a pokemon spawns it add to list his name, id and game object, pokemon script:

 void Start () {
         GameController.instance.AddPokemonToList(GameController.instance.pokemons.Count + 1, this.gameObject.name, this.gameObject);
     }

and then, on the GameController script:

 using UnityEngine;
 using System.Collections;
 using System.Collections.Generic;
 
 [System.Serializable]
 public class Pokemon {
     public int id;
     public string name;
     public GameObject go;
 }
 
 public class GameController : MonoBehaviour {
 
     public List<Pokemon> pokemons;
 
     public static GameController instance;
 
     // Use this for initialization
     void Start () {
     
     }
     
     // Update is called once per frame
     void Update () {
 
     }
     public void AddPokemonToList(int _id, string _name, GameObject _go) {
 
         Pokemon _pokemon = new Pokemon {
 
             id = _id,
             name = _name,
             go = _go
 
         };
 
         instance.pokemons.Add (_pokemon);
 
     }
 }

The console gives me an error on the pokemon script, null reference exeption. What's the problem ?