TYPO3 Core

I have logged in as the backend admin as user "admin" who is automatically the system maintainer.
Now I create a second admin "franz" and give him the admin rights. Now I use the ADMIN TOOLS > Settings > Manage System Maintainers and add "franz" there.
Now I go back into the backend.
Now I am perplexed.

• #1504804727 RuntimeException

*This module "tools_ExtensionmanagerExtensionmanager" is only available as system maintainer *

see https://github.com/TYPO3-Documentation/TYPO3CMS-Exceptions/pull/16/files

TYPO3 should not have allowed to remove myself from the system maintainers. Or at least a warning should have been given. I just did not know that I must also add "admin" go the system maintainers at the moment when I add the user "franz" there.