Creating Multiple Admin Widget Page with Calling One Sidebar

Your code deletes keys from widgets. One of those keys is named ‘class’. When WordPress looks for the ‘class’ key it is missing and PHP reports that.

I think you want this:

$groups_sidebars = array();

foreach ( retrieve_widgets() as $widget ) {
    if ( array_key_exists( 'groups-sidebar', $widget ) )
        $groups_sidebars[] = $widget;
}

$wp_registered_sidebars = $groups_sidebars;