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)}")

What is the table definition? Are you sure that you properly define nullable columns?

Oh 😅 I had defined another Index in the table, so that it's easier to understand what exactly you're joining. Now I removed it and it worked! I didn't really thought about it, because there is also an additional ID in the examples, but I think, the error was that my ID Column was a primary key, the autogenerated too and so it couldn't be generated. But the one in the example is a unique index and not a key. Thanks for giving me the hint!