Asp.net: Nested update panels causing PageRequestManagerServerErrorException

Question

I'm developing an asp.net website. I use nested updatePanels in my code. Piece of code causing the problem:

    <asp:UpdatePanel runat="server" ID="UpdatePanel15" >
                 <Triggers>
                 <asp:AsyncPostBackTrigger ControlID="Button1"/>
                 </Triggers>
                 <ContentTemplate>
                                              
                 <asp:UpdatePanel ID="UpdatePanel10" runat="server">
                 <Triggers>
                  <asp:AsyncPostBackTrigger ControlID="btn_search_" />
                 </Triggers>
                 <ContentTemplate>
                 <asp:DropDownList ID="txtId" runat="server" CssClass="form-control">
                 </asp:DropDownList>
                 </ContentTemplate>
                 </asp:UpdatePanel>
                                                       
                 <asp:TextBox ID="Txt_search_" runat="server" ClientIDMode="Static" CssClass="form-control"></asp:TextBox>
                                                       
                  <asp:Button ID="btn_search_" runat="server" Text="بحث" CssClass="btn btn-primary" OnClick="search_list" />
                                                       
                   <asp:DropDownList ID="DropDowntypes" runat="server" AutoPostBack="false" CssClass="form-control">
                   <asp:ListItem Enabled="true" Text="" Value=""></asp:ListItem>
                                                                </asp:DropDownList>
                                                           
                   <asp:FileUpload ID="FUpload" AllowMultiple="true" runat="server" CssClass="form-control" />


                   <asp:Button ID="Button1" runat="server" Text="إضافة" CssClass="btn btn-primary" OnClick="btnAddDoc_Click" OnClientClick="ShowLoading();" />

            </ContentTemplate>
            </asp:UpdatePanel>

When i click on btn_search_ button every thing goes well, but when i click on Button1 i got this error :Uncaught Sys.WebForms.PageRequestManagerServerErrorException: Sys.WebForms.PageRequestManagerServerErrorException: Object reference not set to an instance of an object.

which i handled by this script: <script type="text/javascript"> Sys.WebForms.PageRequestManager.getInstance().add_endRequest(EndRequestHandler); function EndRequestHandler(sender, args) { if (args.get_error() != undefined) { args.set_errorHandled(true); } } </script>

On the server's event viewer i found this error:

What is causing this exception? How to Solve this issue? Note: Every this is working on localhost the problem appears on the server.