The renderRoot() should be used as a response to a controller. If we use the renderRoot or render() methods incorrectly it can produce warnings such as:
LogicException: Render context is empty, because render() was called outside of a renderRoot()
We are catching a lot of this kind of logs warnings and its related to when cron processes a queue job.
It's hard to reproduce this one, but you can try by creating a lot of notifications for a certain user that has the frequency set to 'Immediate', then running cron multiple times to process these jobs... the warning should appear in the watchdog.
Replace renderRoot with renderPlain
Fixed
12.0
Code (back-end)