C# 在调用方法前，通过检查确定对象是否为null

Question

private void Form1_Load(object sender, EventArgs e)
{
textBox3.Text = 0.ToString();
textBox4.Text = 25.ToString();
textBox5.Text = 3.ToString();
string str = "Provider=Microsoft.Jet.OleDb.4.0;" + "Data Source=" + Application.StartupPath + @"\参数数据库.mdb";
conn = new OleDbConnection(str);
conn.Open();
oda1 = new OleDbDataAdapter("select * from 流体物性", conn);
oda2 = new OleDbDataAdapter("select * from 管件参数", conn);
dt1 = new DataTable();
dt2 = new DataTable();
oda1.Fill(dt1);
oda2.Fill(dt2);
dataGridView1.DataSource = dt1;
dataGridView1.Hide();
dataGridView2.DataSource = dt2;
dataGridView2.Hide();
for (int i = 0; i <= dataGridView1.RowCount; i++)
{
comboBox1.Items.Add(dataGridView1.Rows[i].Cells[0].Value.ToString());
}
}

            第一次运行没问题，当我在加载(再次触发窗体Load事件)第二次的时候就报错

Answer 1