Warning: Creating default object from empty

hi
i am reciving this error on unit page what is the problem :
Warning: Creating default object from empty value in /home/fumahtay/public_html/wp-content/plugins/coursepress/includes/classes/class.shortcodes.php on line 3091

the page is :
http://www.itstar.ir/courses/aenean-auctor-nec-magna-sed-mattis/units/sed-egestas-erat-nec-purus/