TypeError return value of EntityStorageBase::getEntityClass() must be of the type string

Problem/Motivation

#3259941-20: getEntityClass() must be of the type string, null returned →

Steps to reproduce

Drupal 11.1.1
key module 1.x-dev latest as of today
key module 1.19 also affected
symfony mailer lite: 2.0.2

Operation: add a key-override for the smtp username and smtp password of the symfony mailer transport configuration.

The path of the request: /admin/config/development/configuration/key-overrides/add?ajax-form

TypeError: Return value of Drupal\Core\Entity\EntityStorageBase::getEntityClass() must be of the type string, null

Proposed resolution

TBD

Remaining tasks

TBD

User interface changes

TBD

API changes

TBD

Data model changes

TBD