"embed piwik widgets" page refers to site id 1 even if there is no site with the id = 1. So it is impossible to see examples if you delete the first site you enter in piwik configuration.
How to reproduce it. - Add a second site. - Delete the one with the ID = 1. - Go on the "embed piwik widgets" page.
It is due to direct reference to the site with the id = 1 in source:trunk/plugins/Widgetize/Controller.php