As quick & dirty solution I change the url by editing /emails/index.php and change composer to theme, so that the Button New Newsletter points to the theme selection page as is were before the update. But this is also a non update save solution, so I would be the best if there were checkboxes or anything else, with turn off this behaviour.
This would also be the real solution for my theme display problem.
I have actually used a quick and dirty solution.
I removed all rights from the dirs of the included themes, so that they couldn’t be read.
But this isn’t the best pratices but I want our client could only use the custom theme we have created for him.