Returns instance pk even if multiple instances were passed to RichTextField.

def get_instance_id(self, instance):
        " Returns instance pk even if multiple instances were passed to RichTextField. "
        if type(instance) in [list, tuple]:
            core_signals.request_finished.connect(receiver=RichTextField.reset_instance_counter_listener)
            if RichTextField.__inst_counter >= len(instance):
                return None
            else:
                obj_id = self.instance[ RichTextField.__inst_counter ].pk
            RichTextField.__inst_counter += 1
        else:
            obj_id = instance.pk
        return obj_id