Unknown Cause of NullReferenceException?

So I was going through a question I asked awhile ago because I needed a script someone helped me make off of it and found it and was going to use it in my game, but I'm getting a NullReferenceException error.

Here is the error:

 NullReferenceException: Object reference not set to an instance of an object
 GridCreator.Place () (at Assets/Scripts/GridCreator.cs:27)

My code for the GridCreator.cs is this: using UnityEngine; using System.Collections; using UnityEditor;

 public class GridCreator : MonoBehaviour {
 
     [MenuItem("Grid Creator/Create the Grid (9x13)")]
     static void Place()
     {
         // change these to work for you:
         string prefabName = "AnchorPoint"; // case sensitive
         Vector3 gridSize = new Vector3(9, 1, 13); // ie. (10, 1, 5) will generate 10 cubes in the x direction, 1 in the y, and 5 in the z
         Vector3 offset = new Vector3(1, 1, 1); // size of the cubes - default is (1, 1, 1)
         string newCubeName = "AnchorPoint"; // leave blank for default name eg. "Cube(Clone)"
 
 
         GameObject cube = Resources.Load(prefabName, typeof(GameObject)) as GameObject;
 
 
         for (int x = 0; x < gridSize.x; ++x)
         {
             for (int y = 0; y < gridSize.y; ++y)
             {
                 for (int z = 0; z < gridSize.z; ++z)
                 {
                     Vector3 pos = new Vector3(x * offset.x, y * offset.y, z * offset.z);
                     GameObject go = Instantiate(cube, pos, cube.transform.rotation) as GameObject;
 
                     if (newCubeName != "")
                         go.name = newCubeName;
                 }
             }
         }
     }
 }

I am confused as to why it is suddenly not working when it did last time I used it. Only difference is the prefab I'm using. I was using a cube, now I'm using a sphere, but that shouldn't matter.