Airflow 气流分支错误，类型错误：'；非类型'；对象是不可编辑的

我在尝试调用分支操作时遇到以下错误

[2020-01-05 19:11:34,888] {skipmixin.py:78} INFO - Following branch None
[2020-01-05 19:11:34,897] {taskinstance.py:1047} ERROR - 'NoneType' object is not iterable
Traceback (most recent call last):
  File "/root/anaconda3/envs/py3/lib/python3.6/site-packages/airflow/models/taskinstance.py", line 922, in _run_raw_task
    result = task_copy.execute(context=context)
  File "/root/anaconda3/envs/py3/lib/python3.6/site-packages/airflow/operators/python_operator.py", line 142, in execute
    self.skip_all_except(context['ti'], branch)
  File "/root/anaconda3/envs/py3/lib/python3.6/site-packages/airflow/models/skipmixin.py", line 92, in skip_all_except
    for b in branch_task_ids:
TypeError: 'NoneType' object is not iterable
[2020-01-05 19:11:34,900] {taskinstance.py:1076} INFO - All retries failed; marking task as FAILED
[2020-01-05 19:11:35,315] {logging_mixin.py:95} INFO - [[34m2020-01-05 19:11:35,312[0m] {[34mlocal_task_job.py:[0m172} [33mWARNING[0m - [33mState of this instance has been externally set to [1mfailed[0m. Taking the poison pill.[0m
[2020-01-05 19:11:35,321] {helpers.py:319} INFO - Sending Signals.SIGTERM to GPID 25398
[2020-01-05 19:11:35,321] {taskinstance.py:897} ERROR - Received SIGTERM. Terminating subprocesses.

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对于CONVERT_PDF_TO_JPG_任务来说，它的工作方式与预期的一样，发现了问题……这是一个愚蠢的错误，PRE_PROCESS_JPG_任务被创建为BranchPythonOperator而不是常规的PythonOperator，因此它希望从函数返回一个分支id

（旁注：对气流DAG UI团队的建议：热爱UI…但如果不同的操作符用不同的颜色表示，那就太好了。谢谢！）