Please always provide with full error text. The phrases you've shared say nothing without the error line and some other information.

This kind of error comes from some manipulated database. Some topics or posts have been deleted incorrectly and there could be some orphan data in the database. In this case, the ID exists, but other data don't exist. It tries to find other data by ID and nothing finds. So, I'd also recommend Click the following buttons in Dashboard  > Forums > Dashboard admin page:

1. Clean Up
2. Update Topic Statistic
3. Update Forum Statistic
4. Delete All Caches

When you hide a topic from users if it has recent post creates this errors from the recents posts widget.

Please delete these logs, then if you get new one provide us the log info also the row of the warning in the /functions-template.php file.

I noticed my debug.log was almost 4GB (!!!) and it’s full of this php warning: undefined array key.

PHP Warning: Undefined array key “title”
PHP Warning: Undefined array key “topicid”
PHP Warning: Undefined array key “url”

They all point to plugins/wpforo/wpf-includes/functions-template.php file.

Any ideas how troubleshoot this warning? What might be causing it?