title: A control with ID could not be found for the trigger in UpdatePanel
body: <p>I have an update panel that has UpdateMode of Conditional and ChildrenAsTriggers set to false. I only want a few controls to cause an asynchronous postback:</p>
  
  <pre><code>&lt;asp:UpdatePanel ID="updPnlMain" runat="server" UpdateMode="Conditional" ChildrenAsTriggers="false"&gt;
  &lt;ContentTemplate&gt;
  
        // ...
        &lt;asp:Repeater ID="rptListData" runat="server"&gt;
            &lt;ItemTemplate&gt;
                &lt;asp:Button ID="btnAddSomething" runat="server" OnClick="btnAddSomething_Click" /&gt;
            &lt;/ItemTemplate&gt;
        &lt;/asp:Repeater&gt;
        // ...
  &lt;/ContentTemplate&gt;
  &lt;Triggers&gt;
      &lt;asp:AsyncPostBackTrigger ControlID="btnAddSomething" EventName="Click" /&gt;
  &lt;/Triggers&gt;
  &lt;/asp:UpdatePanel&gt;
  </code></pre>
  
  <p>I am getting the following error when I try and load this page:</p>
  
  <pre><code>A control with ID 'btnAddSomething' could not be found for the trigger in UpdatePanel 'updPnlMain'.
  </code></pre>
  
  <p>Since my btnAddSomething control is in a repeater and might not be there right away it acts like it is nonexistent. How can I get around this?</p>