Help instantiating from a for loop.

Hey! Im trying to instantiate a 2d-array of gameobjects.

This is what i have:

 using UnityEngine;
 using System.Collections;
 
 
 public class WorldGen : MonoBehaviour {
     
     public int height;
     public int width;
     private GameObject terrainref;
     private GameObject[] surfacePoints;
 
     private GameObject[,] blocks;
     public GameObject air;
 
     void Start () {
 
         GenerateBlocks ();
         surfacePoints = GameObject.Find ("Terrainref").GetComponent<perlin>().surfacePoints;
     }
     
     void GenerateBlocks() {
         for (int x = 0; x < width; x++){
             for(int y = 0; y < height; y++){
                 blocks[x,y] = Instantiate(air,new Vector3 (x*0.25f,y*0.25f,0),transform.rotation) as GameObject;
             }
         }
     }
 }
 

But all i am getting is "NullReferenceException: A null value was found where an object instance was required" here: blocks[x,y] = Instantiate(air,new Vector3 (x*0.25f,y*0.25f,0),transform.rotation) as GameObject;

I cant find any fault in the code so id appretiate if someone can give me some pointers on how to solve this, thank you!