You're trying to open the panel before it's actually been enhanced as a jQuery Mobile widget.  This is why it's complaining with the "cannot call methods on panel prior to initialization" error.

Notice the "but before all widgets have had an opportunity to enhance the contained markup" from the pagecreate event documentation (http://api.jquerymobile.com/category/events/):

pagecreate

Triggered when the page has been created in the DOM (via ajax or other) but before all widgets have had an opportunity to enhance the contained markup.

You need to use an event that occurs after the widgets have been initialized and have enhanced the page such as pageshow or pageinit.