Button inside Update Panel is not triggered, in asp.net
     <asp:ModalPopupExtender ID="MPE_EditGroup" runat="server" TargetControlID="btnShowPopup" PopupControlID="pnlpopup" CancelControlID="btnCancel" />
     <asp:ToolkitScriptManager ID="ScriptManager1" runat="server" />
     <asp:UpdatePanel ID="UpdatePanel1" runat="server" UpdateMode="Conditional" ChildrenAsTriggers="false">
      <ContentTemplate>
       <asp:Panel ID="pnlpopup" runat="server">
      <asp:ListBox ID="lst_allmembers" DataValueField="FirstName" runat="server" />                       
      <asp:Button ID="btn_Add" runat="server" Text="Add" OnClick="btn_Add_Click" /><asp:ListBox ID="lst_grpmembers" runat="server" />
    <asp:Button ID="btn_remove" runat="server" Text="Remove" />
    <asp:Button ID="btnUpdate" CommandName="Update" runat="server" Text="Update" OnClick="btnUpdate_Click" />
     <asp:Button ID="btnCancel" runat="server" Text="Cancel" /></asp:Panel>
                    </ContentTemplate>
                    </asp:UpdatePanel>

The add button has a event OnClick="btn_Add_Click" 

      protected void btn_Add_Click(object sender, EventArgs e)
        {
            lst_grpmembers.Items.Add(lst_allmembers.SelectedItem.Text);
            lst_allmembers.Items.Remove(lst_allmembers.SelectedItem.Value);
        }

The event is not triggered and I am receiving a error 

    Error: Sys.WebForms.PageRequestManagerServerErrorException: Object reference not set to an instance of an object.

How to trigger the event.
Button inside Update Panel is not triggered, in asp.net
===

     <asp:ModalPopupExtender ID="MPE_EditGroup" runat="server" TargetControlID="btnShowPopup" PopupControlID="pnlpopup" CancelControlID="btnCancel" />
     <asp:ToolkitScriptManager ID="ScriptManager1" runat="server" />
     <asp:UpdatePanel ID="UpdatePanel1" runat="server" UpdateMode="Conditional" ChildrenAsTriggers="false">
      <ContentTemplate>
       <asp:Panel ID="pnlpopup" runat="server">
      <asp:ListBox ID="lst_allmembers" DataValueField="FirstName" runat="server" />                       
      <asp:Button ID="btn_Add" runat="server" Text="Add" OnClick="btn_Add_Click" /><asp:ListBox ID="lst_grpmembers" runat="server" />
    <asp:Button ID="btn_remove" runat="server" Text="Remove" />
    <asp:Button ID="btnUpdate" CommandName="Update" runat="server" Text="Update" OnClick="btnUpdate_Click" />
     <asp:Button ID="btnCancel" runat="server" Text="Cancel" /></asp:Panel>
                    </ContentTemplate>
                    </asp:UpdatePanel>

The add button has a event OnClick="btn_Add_Click" 

      protected void btn_Add_Click(object sender, EventArgs e)
        {
            lst_grpmembers.Items.Add(lst_allmembers.SelectedItem.Text);
            lst_allmembers.Items.Remove(lst_allmembers.SelectedItem.Value);
        }

The event is not triggered and I am receiving a error 

    Error: Sys.WebForms.PageRequestManagerServerErrorException: Object reference not set to an instance of an object.

How to trigger the event.