So either an existing instance is returned, or a new one is created:
return static::$instance ??= new static($app);
But why it ends up null in your case is strange. Can you add debug breakpoints or just output at some of these places so you know at what point null is returned?
Thanks for your response.
I just found the problem… The plugins and mu-plugins folder were copied from an other project and I just saw that there was a older Acorn version included as a mu-plugin …, that caused the conflict.
Sorry to bother you and thanks for your help.