尝试删除Task示例及其子任务时，我遇到了DoesNotExist: Task matching query does not exist。
以下是错误消息：

Traceback (most recent call last):
  File "/Users/thebjko/dev/TrackCracker/venv/lib/python3.11/site-packages/django/db/models/fields/related_descriptors.py", line 218, in __get__
    rel_obj = self.field.get_cached_value(instance)
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/thebjko/dev/TrackCracker/venv/lib/python3.11/site-packages/django/db/models/fields/mixins.py", line 15, in get_cached_value
    return instance._state.fields_cache[cache_name]
           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^
KeyError: 'supertask'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/Users/thebjko/dev/TrackCracker/venv/lib/python3.11/site-packages/django/core/handlers/exception.py", line 55, in inner
    response = get_response(request)
               ^^^^^^^^^^^^^^^^^^^^^
  File "/Users/thebjko/dev/TrackCracker/venv/lib/python3.11/site-packages/django/core/handlers/base.py", line 197, in _get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/thebjko/dev/TrackCracker/crackers/views.py", line 73, in delete
    print(task.delete())
          ^^^^^^^^^^^^^
  File "/Users/thebjko/dev/TrackCracker/venv/lib/python3.11/site-packages/django/db/models/base.py", line 1132, in delete
    return collector.delete()
           ^^^^^^^^^^^^^^^^^^
  File "/Users/thebjko/dev/TrackCracker/venv/lib/python3.11/site-packages/django/db/models/deletion.py", line 512, in delete
    signals.post_delete.send(
  File "/Users/thebjko/dev/TrackCracker/venv/lib/python3.11/site-packages/django/dispatch/dispatcher.py", line 176, in send
    return [
           ^
  File "/Users/thebjko/dev/TrackCracker/venv/lib/python3.11/site-packages/django/dispatch/dispatcher.py", line 177, in <listcomp>
    (receiver, receiver(signal=self, sender=sender, **named))
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/thebjko/dev/TrackCracker/crackers/signals.py", line 10, in reassess_achievement
    if instance.supertask is not None and instance.supertask.completed == False:
       ^^^^^^^^^^^^^^^^^^
  File "/Users/thebjko/dev/TrackCracker/venv/lib/python3.11/site-packages/django/db/models/fields/related_descriptors.py", line 236, in __get__
    rel_obj = self.get_object(instance)
              ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/thebjko/dev/TrackCracker/venv/lib/python3.11/site-packages/django/db/models/fields/related_descriptors.py", line 199, in get_object
    return qs.get(self.field.get_reverse_related_filter(instance))
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/thebjko/dev/TrackCracker/venv/lib/python3.11/site-packages/django/db/models/query.py", line 637, in get
    raise self.model.DoesNotExist(
crackers.models.Task.DoesNotExist: Task matching query does not exist.

到目前为止，我已经弄清楚了错误与信号处理程序有关，因为当
1.处理程序不处理post_delete信号。也就是说，当我从这个处理程序中删除post_delete信号时，

1. instance.supertask is not None
   1.不存在instance.subtasks。
   这些情况是相互排斥的。如果处理程序不处理post_delete，那么是否处理instance.supertask is None并不重要。如果instance.supertask is not None，则示例是否有子任务无关紧要。如果示例有子任务，则instance.supertask is None与否无关紧要。
   对于上述情况，不会引发错误，并且delete()方法成功完成了其工作。该错误仅在处理程序处理post_delete信号时引发，并且如果它有子任务，则没有与示例相关的超级任务。
   在进入if语句时引发错误。
   下面是处理程序：

@receiver([post_save, post_delete], sender=Task)
def reassess_achievement(sender, instance, **kwargs):
    print('*'*100)   # executed
    if instance.supertask is not None and instance.supertask.completed == False:
        print('#'*100)    # not executed
        ...

    # No job for other cases.

但我仍然要执行这段代码每当我删除任务。为什么会出现这个错误，如何调试它？
更新：
如果我在处理程序中打印dir(instance)，它就有'supertask'。为什么我不能使用instance.supertask？