you can use Conditional trigger instead of ContentTemplate.
that will avoid the reflection.

try This >>>>>>>>>>>>

<asp:ScriptManager ID="ScriptManager1" runat="server">
      </asp:ScriptManager>
      <asp:UpdatePanel ID="UpdatePanel1" UpdateMode="Conditional" runat="server">
      <ContentTemplate>
       <asp:TextBox ID="TextBox1" runat="server"></asp:TextBox>
      </ContentTemplate>

      </asp:UpdatePanel>
      <asp:UpdatePanel ID="UpdatePanel2" runat="server"    >
      <ContentTemplate>
           <asp:Button ID="Button1"

          runat="server" Text="Button" />
      </ContentTemplate>
      </asp:UpdatePanel>

You should have used the property ValidationGroup instead of using UpdatePanel which does not make sense at all.

-Eduard