Problem with empty newsletter recipients

[contaoacademy]

Affected version(s)

5.3.8

Description

A customer of mine regularly complains to me about the following error
An exception occurred while executing a query: SQLSTATE[23000]: Integrity constraint violation: 1062 Duplicate entry '1-' for key 'pid_email'

This is due to the fact that he fails to complete the dialog for new newsletter recipients. This results in a gray entry.
If a new recipient is to be created again, the above-mentioned error occurs.

I then simply delete or edit this gray entry and everything works as expected again.
Is there a technical solution to clean this uncompleted entry or is it only possible through intensive training of the user...

Reproducing:

1. Newsletter > «Add new newsletter recipient»
2. do not fill in the form and simply click on another menu item on the left
3. try again to create a new newsletter recipient

[contaoacademy]

Thank you @leofeyer