clearing modal popup extender textbox values

Hi,

I have a gridview in an update panel and a dropdown list. When an item is selected from the dropdown, a modal popup panel gets opened and it contains some textboxes for input. After entering data into the fields in panel, I click 'Ok' button and the data gets bind to the gridview. Everything works nice until here but after opening the modal popup panel again, the field values are not getting erased. I still see the previous values. How do I clear after every 'Ok' click and opening panel again?

Here is my mark up and code behind. Please take a look and guide me. Thanks.

<asp:DropDownList ID="dlChoice" runat="server" AutoPostBack="true"
                            OnSelectedIndexChanged="dlChoice_SelectedIndexChanged"><asp:ListItem Text="Select an option" Value="" /><asp:ListItem>AAA</asp:ListItem><asp:ListItem>BBB</asp:ListItem><asp:ListItem>CCC</asp:ListItem></asp:DropDownList><asp:Panel ID="pnlPopup" runat="server" CssClass="modalPopup" align="center" Style="display: none" Width="800px" Height="250px"><div style="float: left;"><label class="popupLabel">
                                    Date: </label><asp:TextBox BorderStyle="inset" BorderWidth="2px" BorderColor="Black" ID="txtDate" runat="server"></asp:TextBox><br /><br /><label class="popupLabel">
                                    Type: </label><asp:TextBox BorderStyle="inset" BorderWidth="2px" BorderColor="Black" ID="txtType" runat="server"></asp:TextBox><br /><br /><label class="popupLabel">
                                    Form: </label><asp:TextBox BorderStyle="inset" BorderWidth="2px" BorderColor="Black" ID="txtForm" runat="server"></asp:TextBox></div><div style="clear: both; margin: auto;"><br /><asp:Button ID="btnClose2" runat="server" Text="Ok" OnClick="btnClose2_Click" UseSubmitBehavior="False" CausesValidation="false" /></div></asp:Panel></asp:Panel><div id="datagrid"><asp:UpdatePanel ID="UpdatePanel1" runat="server"><ContentTemplate><asp:GridView ID="GridView1" runat="server" HeaderStyle-BackColor="CornflowerBlue" HeaderStyle-Font-Bold="true" HeaderStyle-ForeColor="White" CellPadding="5"></asp:GridView></ContentTemplate><Triggers><asp:AsyncPostBackTrigger ControlID="btnClose2" EventName="Click" /></Triggers></asp:UpdatePanel></div>

protected void Page_Load(object sender, EventArgs e)
		{

            Page.MaintainScrollPositionOnPostBack = true;

            if (!Page.IsPostBack)
            {
                DataTable dt = new DataTable();
                dt.Columns.Add("Date");
                dt.Columns.Add("Type");
                dt.Columns.Add("Form");


                DataRow oItem = dt.NewRow();

                GridView1.DataSource = dt;
                GridView1.DataBind();
                ViewState["dt"] = dt;

              
                
            }
        }
        protected void dlChoice_SelectedIndexChanged(object sender, EventArgs e)
        {
            DropDownList ddl = (DropDownList)sender;

            if (ddl.SelectedValue == "AAA")
            {
                popupExtender.Show();
            }
            else if (ddl.SelectedValue == "BBB")
            {
                popupExtender.Show();
            }
            else if(ddl.SelectedValue=="CCC")
            {
                Modalpopupextender1.Show();
            }
        }

        protected void btnClose2_Click(object sender, EventArgs e)
        {

            DataTable dt = (DataTable)ViewState["dt"];

            DataRow oItem = dt.NewRow();
            oItem[0] = txtDate.Text;
            oItem[1] = txtType.Text;
            oItem[2] = txtForm.Text;
            dt.Rows.Add(oItem);

            GridView1.DataSource = dt;
            GridView1.DataBind();

           
            popupExtender.Hide();

        }