Frequent PHP Warning Since Pop Up Settings Change

Hi,

Just investigating a few (unrelated) issues on a site and found that my error logs are showing an error from the Pop Up Pro plugin very frequently. This started on 31st July, on that day I activated the plugin on a site on this network for the first time and so I presume this is related.

Anyway, any suggestions on how I can stop this? Doesn't seem to be preventing anything from working as far as I can see.

Error:
[Fri Jul 31 13:19:18 2015] [error] [clientxx.xx.xx.xx] PHP Warning: Missing argument 2 for apply_filters(), called in /var/www/vhosts/my.site.co.uk/wp-content/plugins/popover/inc/rules/class-popup-rule-protectedcontent.php on line 59 and defined in /var/www/vhosts/my.site.co.uk/wp-includes/plugin.php on line 177

Cheers!