bu hatayı alıyorum NULL değeri dönüuor galiv nasıl çözebilriirm ?

OleDbConnection con;

con = new OleDbConnection("Provider=Microsoft.Jet.OLEDB.4.0; Data Source=sistem1.mdb");
con.Open();
for (int i = 0; i < dataGridView2.Rows.Count; i++)
{

var val1 = dataGridView2.Rows[i].Cells[0].Value.ToString();
var val2 = dataGridView2.Rows[i].Cells[1].Value.ToString();
var val3 = dataGridView2.Rows[i].Cells[2].Value.ToString();
OleDbCommand cmd = new OleDbCommand();
cmd.Connection = con;
cmd.CommandText = "INSERT INTO sistem2 (bina_isim,maps_x,maps_y) VALUES ('" + val1 + "' , '" + val2 + "' , '" + val3 + "')";
cmd.ExecuteNonQuery();
}
con.Close();