I would say you are probably receiving a WP_Error on first run.
around the $maxitems
and $rss_items
lines, add:
if( ! is_wp_error( $rss ) ) {
$maxitems = $rss->get_item_quantity(3);
$rss_items = $rss->get_items(0, $maxitems);
}
As seen in the example.