I tried applying to only $error and only $old_error, but both appear to be needed.
Log entries:
E_NOTICE (8) classes/urlhelper.php:464 Trying to access array offset on value of type null
1. classes/urlhelper.php(464): ttrss_error_handler(8, Trying to access array offset on value of type null, classes/urlhelper.php, 464, [)
2. classes/rssutils.php(464): fetch([{"url":"https://some.url.rss","login":"","pass":"","timeout":15,"last_modified":"Sat, 31 Aug 2019 15:22:31 GMT"})
3. update.php(235): update_rss_feed(732, 1)
Tiny Tiny RSS
Web-based news feed aggregator, designed to allow you to read news from any location, while feeling as close to a real desktop application as possible.
This program is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.
This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.
You should have received a copy of the GNU General Public License along with this program. If not, see http://www.gnu.org/licenses/.
Copyright (c) 2005 Andrew Dolgov (unless explicitly stated otherwise).
Uses Silk icons by Mark James: http://www.famfamfam.com/lab/icons/silk/