Hi developers and future developers,
I was working through the app dev tutorial (https://docs.nextcloud.com/server/12/developer_manual/app/tutorial.html) and was wondering, why my app doesn’t show anything. At this point in the tutorial you should be suspicious: “and include it at the very top of ownnotes/templates/main.php before the OTHER scripts and styles” 
The solution is in the old ownnotes tutorial app (which unfortunately seems not to work. Version or just nextcloud?): https://github.com/owncloud/app-tutorial/blob/master/templates/main.php
This code has to be in our main.php too. Maybe an Admin can fix that in the Tutorial? Or did I miss an information at some other point?