def update_dropdown_list_slot(self):
        """Keep updating the dropdown list.  Say, don't let the user choose USB devices if none is available
        """
        self.dropdown_widget.clear() # this will trigger dropdown_changed_slot
        self.row_instance_by_index = []
        for i, key in enumerate(self.row_instance_by_name.keys()):
            row_instance = self.row_instance_by_name[key]
            if (row_instance.isActive()):
                self.row_instance_by_index.append(row_instance)
                display_name = row_instance.getName()
                self.dropdown_widget.insertItem(i, display_name)
            row_instance.updateWidget()
Keep updating the dropdown list.  Say, don't let the user choose USB devices if none is available