Error when changing file name "Undefined offset: 3 at /var/www/nextcloud/apps/activity/lib/FilesHooks.php#620"

I am using Nextcloud 12.0.3.
I did not notice because there was no dissatisfaction in operation
When looking at the log, the following error appears when changing the file name.

{"reqId":"wtYHlhLscfZQXWEEQOmo","level":3,"time":"2017-11-29T19:40:36+09:00","remoteAddr":"","user":"root","app":"PHP","method":"MOVE","url":"\/remote.php\/webdav\/aaa.txt","message":"Undefined offset: 3 at \/var\/www\/nextcloud\/apps\/activity\/lib\/FilesHooks.php#620","userAgent":"Mozilla\/5.0 (Windows NT 6.1; Win64; x64; rv:57.0) Gecko\/20100101 Firefox\/57.0","version":""}

What does this error mean?
It seems that there is no problem in operation, but is it a negligible message?

I answered in the wrong topic:

Error should be solved already. Try again with current 12.0.4 and report back to github if there is still this problem.