We'll look into where the PHP notice is coming from. However, that message is only shown because you have the "Error Reporting" setting in the Global Configuration area of Joomla set to something other than "None" (or possibly "System Default"). For a production/live website, you should disable those messages. A PHP notice is just that, something to make you aware of (or to make us aware of), but not a critical error or anything that would likely interfere with normal operation of your site.
I hope that helps explain,
Alex