Object Reference Exception (I'm getting sick of these) :P

For the life of me I can't find the error in this code. Apparently heldBlock is not being set but I can't find the error.

 using UnityEngine;
 using System.Collections;
 
 public class BlockPlacer : MonoBehaviour
 {
 
     private GameManager.Block[] blocks = new GameManager.Block[GameManager.blockSet.Length];
 
     public float reach;
 
     private int i = 0;
 
     private GameManager.Block heldBlock;
 
     private AudioSource[] sounds = new AudioSource[2];
     private AudioSource placeBlock;
     private AudioSource breakBlock;
 
     void Start()
     {
 
         int i = 0;
         foreach (GameManager.Block blck in GameManager.blockSet)
         {
             blocks[i] = blck;
             i++;
         }
 
         sounds = GetComponents<AudioSource>();
         placeBlock = sounds[0];
         breakBlock = sounds[1];
 
 
         heldBlock = blocks[0];
     }
 
     // Update is called once per frame
     void Update()
     {
 
         RaycastHit hitInfo;
 
         //Creating a block
         if (Input.GetButtonDown("Fire1"))
         {
             Ray direction = Camera.main.ScreenPointToRay(new Vector3(Screen.width / 2, Screen.height / 2, 0));
 
             if (Physics.Raycast(direction, out hitInfo, reach))
             {
                 Transform oHit = hitInfo.collider.gameObject.transform;
                 Instantiate(heldBlock.block, new Vector3(oHit.position.x + hitInfo.normal.x, oHit.position.y + hitInfo.normal.y, oHit.position.z + hitInfo.normal.z), Quaternion.identity);
                 placeBlock.Play();
             }
         }
 
         //Deleting a block
         else if (Input.GetButtonDown("Fire2"))
         {
             //Get the block to be deleted
             Ray direction = Camera.main.ScreenPointToRay(new Vector3(Screen.width / 2, Screen.height / 2, 0));
             if (Physics.Raycast(direction, out hitInfo))
             {
                 //Make sure it can be broken
                 if (hitInfo.collider.gameObject.tag == "breakable")
                 {
                     //Break the block
                     Destroy(hitInfo.collider.gameObject);
                     breakBlock.Play();
                 }
             }
         }
 
         //Changing held block
         if (Input.GetAxis("Mouse ScrollWheel") > 0)
         {
             i++;
             if (i <= (blocks.Length - 1))
             {
                 heldBlock = blocks[i];
             }
             else if (i > blocks.Length)
             {
                 i = 0;
                 heldBlock = blocks[i];
             }
         }
         else if (Input.GetAxis("Mouse ScrollWheel") < 0)
         {
             i--;
             if (i >= 0)
             {
                 heldBlock = blocks[i];
             }
             else if (i < 0)
             {
                 i = (blocks.Length - 1);
                 heldBlock = blocks[i];
             }
         }
 
     }
 
     void OnGUI()
     {
         GUI.Box(new Rect(10, 10, heldBlock.blockTex.width, heldBlock.blockTex.height), heldBlock.blockTex, GUIStyle.none);
     }
 
 }

the error is being thrown in OnGUI()

It's a lot of code but you need it all if you're gonna find it.

I'm getting three separate errors. The first one happens about 5 five times before I stop the game.

NullReferenceException: Object reference not set to an instance of an object BlockPlacer..ctor ()

the second and third are both logged only once

NullReferenceException: Object reference not set to an instance of an object WorldGeneration.Start () (at Assets/Scripts/WorldGeneration.cs:15)

NullReferenceException: Object reference not set to an instance of an object (wrapper stelemref) object:stelemref (object,intptr,object) BlockPlacer.Start () (at Assets/Scripts/BlockPlacer.cs:25)