Sander,
That message is just a PHP Warning, and not a real 'error'. We will have that message fixed in the next release so that it no longer appears. For now, we're recommend setting the "Error Reporting" setting in the Joomla -> Global Configuration area to "None". That will hide the message from appearing at all.
I hope that helps. If the solution above doesn't get rid of the problem, let us know. We can provide you with a minor code change to fix the issue as well, but it shouldn't be necessary.
Thanks,
Alex