Programmatically Set SkinId Properties For Controls Within UpdatePanel?

[Joey]

In my app I have many master/content pages. On each content page I set
controls in the Page_PreInit event. This was working fine until I
added AJAX and an UpdatePanel to one of my content pages. Now for all
of the controls contained within UpdatePanel tags, I get "Object
reference not set to an instance of an object" at runtime, when trying
to programmatically set SkinIds.

Can anyone tell me why this is happening? Better, can anyone tell me
how to reference the controls in the code behind so that I don't get
the error?

JP