Catchable fatal error

I use All-in-One Event Calendar and when i try to make a collapse menu with “Collapsing Categories” plugin, nothing appears.
Moreover when , in the settings of “Collapsing Categories” , check “Show empty categories” the following message appears:

Catchable fatal error: Object of class WP_Error could not be converted to string in /home/ergolhpt/public_html/wp-content/plugins/collapsing-categories/collapscatlist.php on line 61

lines 60 and 61 are :

} elseif ($cat->taxonomy==’category’) {
$link = “<a href='”.get_category_link($the_cat).”‘ “;

can anybody help me please???

http://wordpress.org/extend/plugins/all-in-one-event-calendar/