Should throw an error if no theme is useable

[Olli]

If no theme is available because its not created it should be displayed an error with a hint, that a theme should be created.
Now one get a white screen and a error in the error.log file:
Call to a member function next() on a non-object in */composer/vendor/bit3/contao-twig/src/ContaoTwig/ContaoTwig.php on line 111