c# getting NullReferenceException when retrieving data from a cell in DataGridView

This is my code:

private void CostList_Load(object sender, EventArgs e)
{
    // TODO: This line of code loads data into the 'lSEStockDataSet.CostPrice' table. You can move, or remove it, as needed.
    this.costPriceTableAdapter.Fill(this.lSEStockDataSet.CostPrice);

    con = new System.Data.SqlClient.SqlConnection();
    con.ConnectionString = "Data Source=tcp:SHEN-PC,49172\\SQLEXPRESS;Initial Catalog=LSEStock;Integrated Security=True";
    con.Open();

    DataGridView datagridview1 = new DataGridView();
    String retrieveData = "SELECT CostID, SupplierName, CostPrice FROM CostPrice WHERE PartsID ='" + textBox1.Text + "'";
    SqlCommand cmd = new SqlCommand(retrieveData, con);
    int count = cmd.ExecuteNonQuery();
    SqlDataReader dr = cmd.ExecuteReader();
    DataTable dt = new DataTable();
    dt.Load(dr);
    dataGridView1.DataSource = dt;
    con.Close();

}

private void button1_Click(object sender, EventArgs e)
{

    if (dataGridView1.Rows.Count > 0)
    {   
        int nRowIndex = dataGridView1.Rows.Count-1;


        if (dataGridView1.Rows[nRowIndex].Cells[2].Value != null)
        {
            textBox2.Text = Convert.ToString(dataGridView1.Rows[nRowIndex].Cells[2].Value);
        }
        else
        {
            MessageBox.Show("NULL");
        }
    }
}

It shows NULL when i clikc the button, what is the problem here? I have 3 columns there, i want to get the data of the 3rd column of the last row, but it shows NULL but there is data in the specified cell. Anyone knows how to solve this problem?