将数据插入数据库后如何更新DataGridView？

Question

如何解决此问题？

  

System.NullReferenceException：'对象引用未设置为对象的实例。'

public void show_list()
{
    objDataAdapter.SelectCommand = new SqlCommand();
    objDataAdapter.SelectCommand.Connection = objConnection;
    objDataAdapter.SelectCommand.CommandText =
        "SELECT T,W " + "FROM R";
    objDataAdapter.SelectCommand.CommandType = CommandType.Text;
    objConnection.Open();
    objDataAdapter.Fill(objDataSet, "R");
    objConnection.Close();
    dataGridView1.AutoGenerateColumns = true;
    dataGridView1.DataSource = objDataSet;
    dataGridView1.DataMember = "R";
    objDataAdapter = null;
}

private void button1_Click(object sender, EventArgs e)
{
    objConnection.Open();
    SqlCommand objcmm = new SqlCommand();
    objcmm.Connection = objConnection;
    objcmm.CommandText = "INSERT INTO R" +
        "(T,W)" +
        "values(@t,@w);";
    objcmm.Parameters.AddWithValue("@t", txt1.Text);
    objcmm.Parameters.AddWithValue("@w", txt2.Text);
    objcmm.ExecuteNonQuery();
    objConnection.Close();
 //   MessageBox.Show("Done");
    txt1.Text = null;
    txt2.Text = null;
      show_list();
}