NullReferenceException when trying to programatically populate cubes

I've been trying to build a 2d tile map generator, and I'm running into a "NullReferenceException: Object reference not set to an instance of an object" error. The problem is being pointed to here.

         grass = (GameObject)Instantiate(Resources.Load("GrassPrefab"));
         for (int i = 0; i < TestMap.map.getSize(0); i++) //error here
         {
             for (int j = 0; j < TestMap.map.getSize(1); j++)
             {
                 if(TestMap.map[j, i] == 0)
                 {
                     Instantiate(grass, new Vector3(i, j, 10), Quaternion.identity);
                 }
             }
         }

I suspect that the problem is actually how my map loads from a file here:

 enter code herevoid LoadMap()
     {
         sourceFile = new FileInfo("Assets/Resources/testmap.txt");
         reader = sourceFile.OpenText();
 
         while (true)
         {
             string[] split = reader.ReadLine().Split(new char[1] { ',' });
             List<int> numbers = new List<int>();
             int parsed;
 
             foreach (string n in split)
                 if (int.TryParse(n, out parsed))
                     numbers.Add(parsed);
 
             map.Add(numbers);
 
             if (reader.EndOfStream)
                 break;
         }
     }

Are my suspicions true, am I loading the txt file wrong? (Located in Assets/Resources folder where my prefab is) Or do I have the wrong idea as to how a prefab works; do I need a list or array of GameObjects to load each one?

Edit: further information on TestMap.map:

TestMap.map is a 2d list as defined: public static Map2d map;

 //Map2d:
 
 public class Map2d<T>
 {
     List<List<T>> matrix;
 
     public void Add(IEnumerable<T> row)
     {
         List<T> newRow = new List<T>(row);
         matrix.Add(newRow);
     }
 
     public T this[int x, int y]
     {
         get { return matrix[y][x]; }
     }
 
     public int getSize(int x)
     {
         if(x == 0)
             return matrix.Count;
         else
             return matrix[x].Count;
     }
 }