The thing you want to instantiate is null. UnityEngine.Object.CheckNullArgument

public static int gridWidth =10;
public static int gridHeight =20;

// Use this for initialization
void Start () {

    SpawnNextTetrimino();

}

// Update is called once per frame
void Update () {

}

public void SpawnNextTetrimino()
{
    GameObject nextTetrimino = (GameObject)Instantiate(Resources.Load(GetRandomTetrimino(), typeof(GameObject)), new Vector2(5.0f, 20.0f), Quaternion.identity);
    

}

public bool CheckIsInsideGrid (Vector2 pos){
	return ((int)pos.x >= 0 && (int)pos.x < gridWidth && (int)pos.y >= 0);
}

public Vector2 Round (Vector2 pos){
	return new Vector2 (Mathf.Round (pos.x), Mathf.Round (pos.y));

}

string GetRandomTetrimino()
{

    int randomTetrimino = Random.Range(1, 8);

    string randomTetriminoName = "Prefabs/Tetromino_T";

    switch (randomTetrimino)
    {
        case 1:
            randomTetriminoName = "Prefabs/Tetromino_T";
            break;
        case 2:
            randomTetriminoName = "Prefabs/Tetromino_Long";
            break;
        case 3:
            randomTetriminoName = "Prefabs/Tetrimino_Square";
            break;
        case 4:
            randomTetriminoName = "Prefabs/Tetrimino_J";
            break;
        case 5:
            randomTetriminoName = "Prefabs/Tetrimino_S";
            break;
        case 6:
            randomTetriminoName = "Prefabs/Tetrimino_Z";
            break;
        case 7:
            randomTetriminoName = "Prefabs/Tetomino_L";
            break;

    }

    return randomTetriminoName;

}

}

Are the prefabs in a Resources folder? e.g. “Resources/Prefabs/Tetromino_T”

http://docs.unity3d.com/ScriptReference/Resources.html

http://docs.unity3d.com/Manual/SpecialFolders.html