update statement giving two differen exceptions

string new_val = new_value_txt.Text.ToString();

int rowindex = dataGridView3.CurrentCell.RowIndex;
int row = dataGridView3.CurrentCell.RowIndex;

string row_index_value = dataGridView3["ID", row].Value.ToString();


if (String.IsNullOrEmpty(new_value_txt.Text.Trim()) && String.IsNullOrEmpty(new_value_txt.Text.Trim()))
{

    MessageBox.Show("Please Enter a New Value..", "Enter a Value", MessageBoxButtons.OKCancel);

}


OleDbCommand cmd = new OleDbCommand("update Purchases SET " + name_of_column + " ='" + new_val + "' WHERE " + name_of_column + " = '" + cellvalue + "' AND  ID  = '" + row_index_value + "'   ", con);

con.Open();
cmd.ExecuteNonQuery();
con.Close();

MessageBox.Show("updated Successfully!!");