I now switched to use the DAO for interaction with TornadoFX, because it seems to work while writing. But when I execute the program, I get an IAE: Parameter specified as non-null is null: method org.jetbrains.exposed.dao.EntityClass.wrap, parameter id. But in the docs there is nothing in the examples about having to use an Id apart from the definition of the class. And in the line above I access the database table with the DSL and there it works, so the database isn't empty.
println("DSL: ${Kreuzungen.selectAll().toList().get(0)}")
println("Element 1: ${Kreuzung.all().toList().get(0)}")