Create Craft 3 Plugin - Call to a member function getVersion() on null
Shame on me, but maybe still interesting to others who have this problem: I'm using vagrant and the path of my extension was NOT mounted to the vagrant box. That's all. So it simply could not find the files. Maybe a warning by craft would be nice, that it can not load the plugin files

Though I do not understand, why it found the composer.json to read the basic information, which showed up in the Plugins section of Craft...