this._pagerequestManager is null or not an object

If you run into the JavaScript exception while attempting to add animations to your controls there is a setting in your web.config that you need to update. Open your web.config and locate the tag

<xhtmlConformance mode="Legacy"/> and replace with <xhtmlConformance mode="Transitional"/>.

Full Page Processed on Partial PostBack

One odd thing you need to keep in mind when using and the AjaxControlToolkit is that when the UpdatePanel is used to do an asynchronous call back to the server the entire page state is sent back to the server and the entire page is reprocessed. This means that everything in your Page_Load and other initialization methods will be called again.