[kiwidude]

I believe you will need to change the point at which you return the tuple. Of course now we are creating a ConfigWidget class instance, and doing stuff inside the __init__() function of it, whereas previously you were directly inside the config_widget() function itself.

So if I were you I would not put the code in ConfigWidget() at all. Instead put the check in the config_widget() function in your InterfaceActionBase class.
ie.

def config_widget(self):
    if self.not_valid_to_configure()
        return ('My message', 'My details')
    from calibre_plugins.kindle_collections.config import ConfigWidget
    return ConfigWidget(self.plugin_action_)