模式弹出AJAX在刷新时退出

这是一个情况，我有一个AJAX模式弹出窗口，在我的面板内是两（2）个连接的下拉列表。一个代表欧洲大陆，另一个代表各国。一个例子是，当用户选择亚洲时，国家的下拉列表中应该有数据。这是我的模式弹出窗口和面板的代码

  <asp:ModalPopupExtender ID="Modalpopupextender1" runat="server" TargetControlID="ShowPopUpButton"
        PopupControlID="pnlpopup" CancelControlID="CancelButton" BackgroundCssClass="modalBackground">
     </asp:ModalPopupExtender>
  <asp:Panel ID="pnlpopup" runat="server" BackColor="White" Height="269px" Width="400px"
        OnLoad="pnlpopup_Load">
    <tr>
                <td align="right">
                    Continent:
                </td>
                <td>
                    <asp:DropDownList ID="ContinentDownList" runat="server" 
                        onselectedindexchanged="ContinentDropDownList_SelectedIndexChanged" AutoPostBack="true">
                    </asp:DropDownList>
                </td>
            </tr>
            <tr>
                <td align="right">
                    Country:
                </td>
                <td>
                    <asp:DropDownList ID="CountryDropDownList" runat="server">
                    </asp:DropDownList>
                </td>
            </tr>
      </asp:Panel>

现在我的问题是，当我的模式弹出窗口加载时，当我选择一个大陆时，国家的下拉列表不会加载。当我在ContinentDropDown中插入AutoPostBack="true"时，模式弹出窗口会刷新并退出。我花了很长时间调试并知道如何解决这个问题。救命

这是后面的代码

 protected void Page_Load(object sender, EventArgs e)
    {
        if (!Page.IsPostBack)
        {
            LoadContinent();
            LoadCountry();
        }
    }
  public void LoadContinent()
    {
        using (SqlConnection con = new SqlConnection(ConfigurationManager.ConnectionStrings["ConnectionString"].ConnectionString))
        {
            using (SqlCommand com = new SqlCommand("Reader.usp_LoadContinentDropDownList", con))
            {
               com.CommandType = CommandType.StoredProcedure;
                con.Open();
                try
                {
                   SqlDataReader dr = com.ExecuteReader();
                   OwnerGroupDropDownList.DataSource = dr;
                    OwnerGroupDropDownList.DataTextField = "fld_Description";
                    OwnerGroupDropDownList.DataValueField = "fld_ContinentID";
                    ContinentDropDownList.DataBind();
                }
                catch (SqlException)
                {
                    Response.Write("<script>alert('The database has encountered an error. Please try again')</script>");                    }
                catch (Exception)
                {
                    Response.Write("<script>alert('The database has encountered an error. Please try again')</script>");                    }
            }
        }
        LoadContinentDropDownList.Items.Insert(0, new ListItem("<Select Person Group>", "0")); 
    }

         public void LoadCountry()
    {
        using (SqlConnection con = new SqlConnection(ConfigurationManager.ConnectionStrings["ConnectionString"].ConnectionString))
        {
            using (SqlCommand com = new SqlCommand("Reader.usp_LoadCountryDropDownList", con))
            {

                com.CommandType = CommandType.StoredProcedure;
                com.Parameters.Add(new SqlParameter("@fld_ContinentId", SqlDbType.Int));
                com.Parameters["@fld_ContinentId"].Value = ContinentDropDownList.SelectedValue;
                con.Open();
                try
                {
                    SqlDataReader dr = com.ExecuteReader();
                    OwnerDropDownList.DataSource = dr;
                    OwnerDropDownList.DataTextField = "fld_Description";
                    OwnerDropDownList.DataValueField = "fld_CountryID";
                    CountryDownList.DataBind();
                }
                catch (SqlException)
                {
                    Response.Write("<script>alert('The database has encountered an error. Please try again')</script>");                    }
                catch (Exception)
                {
                   Response.Write("<script>alert('The database has encountered an error. Please try again')</script>");                    }
            }
        }
         CountryDropDownList.Items.Insert(0, new ListItem("<Select Person>", "0")); 
    }
      protected void ContinentDropDownList_SelectedIndexChanged(object sender, EventArgs e)
    {
        LoadContinent();
        LoadCountry();
    }