Decided to look into it. Seems it was using a deprecated cache file. I am using 7.4.2 and to do this to fix it:
In admin/edittopicfeed.php find:
(file_exists("{$config['FULL_PATH']}/cache/forum_list.php")) ? include("{$config['FULL_PATH']}/cache/forum_list.php") : $tree = array() ;
and replace with
(file_exists("{$config['FULL_PATH']}/cache/forum_cache.php")) ? include("{$config['FULL_PATH']}/cache/forum_cache.php") : $tree = array() ;