ModalPopupExtender disappers after dropdownlist's selected index change

I want to get some value from dropdown's selected index change and bind them to textox or other control but problem is that popup disappear after selected index changed. Need Solution. code is ``` <ajaxToolkit:ToolkitScriptManager ID="ToolkitScriptManager1" runat="server"> </ajaxToolkit:ToolkitScriptManager> <div> <div> <div> <asp:Button ID="Button1" runat="server" onclick="Button1_Click" Text="Button" /> <asp:ModalPopupExtender ID="Button1_ModalPopupExtender" runat="server" Drag="True" DropShadow="True" DynamicServicePath="" Enabled="True" OkControlID="btncancel" PopupControlID="Panel1" TargetControlID="Button1"> </asp:ModalPopupExtender> </div> </div> </div> <asp:Panel ID="Panel1" runat="server"> first name<asp:TextBox ID="TextBox5" runat="server"></asp:TextBox> <br /> country <asp:DropDownList ID="DropDownList1" runat="server" AutoPostBack="True" onselectedindexchanged="DropDownList1_SelectedIndexChanged"> <asp:ListItem>select</asp:ListItem> <asp:ListItem>india</asp:ListItem> <asp:ListItem>pakistan</asp:ListItem> <asp:ListItem>usa</asp:ListItem> <asp:ListItem>england</asp:ListItem> </asp:DropDownList> <asp:RequiredFieldValidator ID="RequiredFieldValidator1" runat="server" ControlToValidate="DropDownList1" ErrorMessage="*" InitialValue="select"></asp:RequiredFieldValidator> <br /> selected country<asp:TextBox ID="TextBox9" runat="server"></asp:TextBox> <br /> <asp:Button ID="Button2" runat="server" onclick="Button2_Click" Text="Submit" /> <input id="btncancel" type="button" value="Cancel" /> </asp:Panel> ``` And code behind is ``` protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e) { TextBox9.Text = DropDownList1.SelectedValue; } ```