Object reference not set to an instance of an object

hi i am quite new to unity, i am trying to make a 2d game. Anyway, i trying to make my ship to shoot the bullet out. The code work but when i try to follow the tutorial about Inter-object communication ( basically separate the code out) i keep getting the error: NullReferenceException: Object reference not set to an instance of an object scrip.Update () (at Assets/scrip/Player_controller.cs:38)

well it point to: game_state.instance.Spawn_bullet(ship_gun.position);

this is my player_controller scrip using UnityEngine; using System.Collections;

 public class scrip : MonoBehaviour {
     public float speed = 1f;
     Transform ship_gun;
     public GameObject player_bullet;
     // Use this for initialization
     void Start () {
 
         ship_gun = transform.Find("gun");
     }
     
     // Update is called once per frame
     void Update () {
         float deltaX = 0f, deltaY = 0f;
         if (Input.GetKey(KeyCode.W))
         {
             deltaY -= 1f;
         }
         if (Input.GetKey(KeyCode.S))
         {
             deltaY += 1f;
         }
         if (Input.GetKey(KeyCode.D))
         {
             deltaX += 1f;
         }
         if (Input.GetKey(KeyCode.A))
         {
             deltaX -= 1f;
         }
         
         // fire bullet
         if (Input.GetKey(KeyCode.Space))
         {
             game_state.instance.Spawn_bullet(ship_gun.position);
         }
 
         //update moverment
         transform.Translate(new Vector3(deltaY, deltaX, 0f) * Time.deltaTime * speed);
 
 
     }
 }

and this is my game_state scrip

 using UnityEngine;
 using System.Collections;
 
 public class game_state : MonoBehaviour
 {
     [SerializeField]
     GameObject player_bullet;
     public GameObject Spawn_bullet(Vector3 _position)
     {
         return (GameObject)Instantiate(player_bullet, _position, Quaternion.identity);
     }
 
     public static game_state instance = null;
     void alwake ()
     {
         if (instance != null)
         {
             Debug.LogError("all found");
         }
         instance = this;
     }
     
 }
 

Well i am quite unsure what is wrong with it, the tutorial is unclear about where to add the game_state scrip to so i am quite unclear about the scrip is wrong or i am doing something else wrong

Thank for helping me :)