Пожалуйста помогите, я не понимаю в чем проблема, почему вылетает ошибка? 
 Код
using UnityEngine; 
 using System.Collections; 
 public class GameController : MonoBehaviour { 
   
  public GameObject hookPrefab; 
  Grid grid; 
  void Start ()  
  { 
   grid = GetComponent<Grid>(); 
   CreateHook(); 
  } 
   
  public void CreateHook() 
  { 
   int action = Random.Range (0, 2); 
   Hook hook = Instantiate (hookPrefab, new Vector3((action * -21) + 16, 8, 0), Quaternion.identity) as Hook; 
   hook.SetParams(5, action); 
  } 
 }
 NullReferenceException: Object reference not set to an instance of an object 
 GameController.CreateHook () (at Assets/Scripts/GameController.cs:25) 
 GameController.Start () (at Assets/Scripts/GameController.cs:18)