Undefined offset: 3 at /var/www/nextcloud/apps/activity/lib/FilesHooks.php#620

I’m currently running version 12 on a Ubuntu 16.04 LTS server
I have been seeing this error messages here and there in the logfile.

Error	PHP	Undefined offset: 3 at /var/www/nextcloud/apps/activity/lib/FilesHooks.php#620

According to this link this should solve the issue:

But where is the file located? I don’t know where to find it.

This fix should already be in the current version. Try again with NC 12.0.4 and report to github if there is still an issue.