Null Reference Exception For Blocks C#

In a game that I'm creating in Unity, there is an issue with the game running. In the code that I'll put here, you'll see that I am attempting to call certain rounds and build a grid of blocks. Before, the game was running fine and it was finding the object that I was calling with no issues what so ever. Now, however, even though I haven't changed much if anything, it's throwing me a Null Reference Exception stating that Object Reference is not Set to an Object, even though it clearly is. Why is this happening? I'm not going to post the full script, because there's too much, but I'll post just the area where the Error is appearing. The variable for "firstBlock" IS being created as a variable in my public scope and is declared as a GameObject. So, I'm not sure why it's throwing me a Null Reference stating that there is no object attached to this section of code. Also, in my unity Scene, I've set the firstBlock to a GameObject I created in the game, so that's also not the issue.

 #region createWorld
     void createWorld (int i) {
         int r = i+1;
         Round thisRound = new Round();
         thisRound.roundNumber = r;
 
         if (r == 1) {
             //Create a grid of blocks that chooses a random number and assigns an asset to that number
             for (int x = 0; x<gridWidth; x++) {
                 
                 for (int z = 0; z<gridHeight; z++) {
                     
                     int chooseBlock = Random.Range(0, 3);
                     gameBoard.grid[x,z] = Instantiate(firstBlock, transform.position, transform.rotation) as GameObject;
                     //NULL REFERENCE ISSUE HERE ^^^
                     gameBoard.grid[x,z].AddComponent<Block>();
                     gameBoard.grid[x,z].transform.parent = transform;
                     gameBoard.grid[x,z].transform.localPosition = new Vector3 (x-4.5f, 1, z-4.5f);
                     gameBoard.grid[x,z].renderer.material.mainTexture = blockTex[chooseBlock];
                     gameBoard.grid[x,z].transform.localScale = new Vector3 (.95f, .95f, .95f);
                 }
             }
         }
         else {
             for (int x = 0; x<gridWidth; x++) {
                 
                 for (int z = 0; z<gridHeight; z++) {
                     
                     int chooseBlock = Random.Range(0, 5);
                     gameBoard.grid[x,z] = Instantiate(firstBlock, transform.position, transform.rotation) as GameObject;
                     gameBoard.grid[x,z].AddComponent<Block>();
                     gameBoard.grid[x,z].transform.parent = transform;
                     gameBoard.grid[x,z].transform.localPosition = new Vector3 (x-4.5f, 1, z-4.5f);
                     gameBoard.grid[x,z].renderer.material.mainTexture = blockTex[chooseBlock];
                     gameBoard.grid[x,z].transform.localScale = new Vector3 (.95f, .95f, .95f);
                 }
             }
         }
     }
     #endregion
 
     #region roundControl
     void roundControl () {
         createWorld (0);
     }
     
     #endregion
 
     #region Menu Controls
     void OnEnable() {
         startMenu();    
     }
     
     public void playClicked() {
         activateScreens(menuScreen.Level);
     }
     
     /*
     public void instructionsClicked() {
         activateScreens(menuScreen.Instructions);
     }
     */    
     public void startMenu() {
         activateScreens(menuScreen.Menu);    
     }
     
     void activateScreens(menuScreen changeScreen) {
         
         screen = changeScreen;
         
         mainMenu.SetActive(false);
         //mainInstruction.SetActive(false);
         mainGame.SetActive(false);
         
         switch(changeScreen) {
         case menuScreen.Menu:
             luckyBackground.renderer.material.mainTexture = firstMenu;
             mainMenu.SetActive(true);
             break;
         //case menuScreen.Instructions:
             //luckyBackground.renderer.material.mainTexture = firstMenu;
             //mainInstruction.SetActive(true);
         //break;
         case menuScreen.Level:
             roundControl();
             luckyBackground.renderer.material.mainTexture = playGame;
             mainGame.SetActive(true);
             break;
         }
         
     }    
     #endregion