I deleted a Visulaization and an UML class-diagram that I couldn't get to run and messed around with some libraries, but I have no clue what really fixed it for me. Thanks anyway for your quick reply!
Late to the Party, but did you find a fix for the Problem? I have a similiar issue with "Internal error:System.NullReferenceException: Object reference not set to an instance of an object. at" just with other locations(?) afterwards