我正在使用以下代码来检查以前添加到复选框列表中的db表中的值，但是在这里出现“对象引用未设置为对象实例”错误：

ListItem currentCheckBox = chkbx.Items.FindByValue(rdr["MemberID"].ToString());

这是代码，谢谢您的帮助！

SqlDataReader rdr = null;
SqlConnection conn = new SqlConnection(GetConnectionString());
SqlCommand cmd5 = new SqlCommand("SELECT MemberID FROM ProjectIterationMember WHERE ProjectIterationID IN (SELECT ProjectIterationID FROM Iterations WHERE ProjectID = '" + proj_id + "')", conn);

try
{
    conn.Open();
    rdr = cmd5.ExecuteReader();

    CheckBoxList chkbx = (CheckBoxList)FindControl("project_members");
    while (rdr.Read())
    {
        ListItem currentCheckBox = chkbx.Items.FindByValue(rdr["MemberID"].ToString());
        if (currentCheckBox != null)
        {
            currentCheckBox.Selected = true;
        }
    }
}
finally
{
    if (rdr != null)
    {
        rdr.Close();
    }

    if (conn != null)
    {
        conn.Close();
    }
}