TYPO3 Core

1. Create a form with an email finisher and define one or more default recipients
2. Insert the form and submit it. Works fine.
3. Override the recipients by using the "Override finisher settings" option.
4. Submit the form. Works fine.
5. Remove the overriden recipients again, but keep "Override finisher settings" enabled, as there are more settings to be overriden.
6. Submit the form. This results in an error `The option "recipients" must be set for the EmailFinisher.`.

This is because the empty recipient override is processed and returns an empty string, which is then written into the final finisher configuration. This way, the empty receiver list overrides the default receiver list.