As we always make sure the parent is a widget class we don't have to check anymore.
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I think you can go ahead and simply remove these efl_isa calls.
The parent (if set) is always a efl.ui.widget. Or the code at A would have already crashed. More importantly: in the .eo files parent is annotated as widget, and in all signatures as well, so this was never meant to work with a none efl.ui.object.