Blank Pages when clicking 'Done' or 'Next' in Coursepress Pro

Hi there, I have a range of courses in my new Coursepress site and I'm now getting blank pages whenever I click to the next page of the course.

So, I'll click the unit name, and the 1st page will load up fine. But then I click 'Next' and it shows a blank page. I've turned on Debugging and it also shows the following error:

Notice: Undefined variable: wp in /home/fanfission/public_html/wp-content/plugins/coursepress/includes/classes/class.course.unit.php on line 561

Notice: Trying to get property of non-object in /home/fanfission/public_html/wp-content/plugins/coursepress/includes/classes/class.course.unit.php on line 561

I've enabled support access so that you can have a look.

Your help would be appreciated.
Colin