Replace Swing GUI binding Java class with Kotlin class

I want to replace the default Java class that gets generated with a Swing GUI form with a Kotlin version of the same. Just transforming the given Java class to a Kotlin version does not help. For example: it replaces a statement like "public JPanel myPanel" in the Java version with "

var panelMain: JPanel? = null

That does not work. The binding wizzard does not recognize the transformed Kotlin class. So, I guess my question is: how to build the field binding properly?