Asp.net - User Control Not Instanciated On A Postback?

I'd like to set the trigger of an UpdatePanel to a user control outside the UpdatePanel on the same page . The user control is added at design time, not at runtime. If I statically declare the trigger, I get an error "A control with ID 'xx' cannot be found for the trigger in UpdatePanel". I tried to add the trigger at runtime in Page_Init or Page_Load, but it fails with the user control being null, although it has ViewState enabled. Has someone an idea on how to solve this?

Here is the code of the user control:

<%@ Control Language="C#" AutoEventWireup="true" CodeFile="ComponentDropDownControl.ascx.cs" Inherits="ComponentDropDownControl" EnableViewState="true" %>

[code...]


In the Page_Load function of the aspx page, the following lines work at first time, but fail on the first PostBack (line 2, CustomComponentDropDown is null).AsyncPostBackTrigger trigger = new AsyncPostBackTrigger(); trigger.ControlID = CustomComponentDropDown.UniqueID.ToString();
UpdatePanel1.Triggers.Add(trigger);