how to use custom classes for the fields on refclass in r

solution found here
https://grokbase.com/t/r/r-sig-gui/115dbbfqmx/r-gui-gwidgetsrgtk2-problem-with-gradio-handler-in-a-proto-object

problem is that:

internal problem in as(): “__main__.Istree” is(object, "python.builtin.object") is TRUE, but the metadata asserts that the 'is' relation is FALSE