B4J Question fx.Msgbox2 error inside a class module

I'm trying to call fx.Msgbox2 from inside a class module but I keep getting a NullPointerException

I've tested it outside of the class module and it works fine. What extra do we have to do to get it to work inside a class module?

fx.Msgbox2(MainForm, "The input data was not in a recognizeable format!","Input Error", "OK", "", "", fx.MSGBOX_ERROR)

Any help appreciated, even if it's a pointer to documentation that covers this scenario.