private void btnRoll_Click(object sender, EventArgs e)
    {
        lbDice.Items.Clear();
        int[] rolls = new int[13];
        for (int i = 1; i < numTxt.Value; i++) {
            int index = new Random().Next(1, 7) + new Random().Next(1, 7);
            //MessageBox.Show(index + ""); THIS LINE IS REQUIRED
            rolls[index] += 1;
        }
        updateList(rolls);
    }


    public void updateList(int[] rolls)
    {
        for (int i = 1; i < rolls.Length; i++)
        {
            lbDice.Items.Add("" + i + "  " + rolls[i]);
        }
    }