[Python] airflow 'NoneType' object has no attribute 'is_paused',how to fix it?

I am new to airflow and I just follow the tutorial to run a dag. Actually I did it successfully, but the problem is when I try to pause the dag by inputing command line like airflow pause dag_id,it throws AttributeError: 'NoneType' object has no attribute 'is_paused'

And the Traceback is:

File "/anaconda3/bin/airflow", line 32, in <module>
args.func(args)
File "/anaconda3/lib/python3.6/site-packages/airflow/utils/cli.py", line 74, in wrapper
return f(*args, **kwargs)
File "/anaconda3/lib/python3.6/site-packages/airflow/bin/cli.py", line 365, in pause
set_is_paused(True, args, dag)
File "/anaconda3/lib/python3.6/site-packages/airflow/bin/cli.py", line 379, in set_is_paused
dm.is_paused = is_paused


So I check the python file in the output directory, however, it was coded by airflow and I don't know how to fix it?!!!

SHOW:~ macbook$ airflow list_dags
[2018-09-07 21:31:38,262] {__init__.py:51} INFO - Using executor SequentialExecutor
[2018-09-07 21:31:38,381] {models.py:258} INFO - Filling up the DagBag from /Users/macbook/airflow/dags


-------------------------------------------------------------------
DAGS
-------------------------------------------------------------------
example_bash_operator
example_branch_dop_operator_v3
example_branch_operator
example_http_operator
example_kubernetes_executor
example_kubernetes_operator
example_passing_params_via_test_command
example_python_operator
example_short_circuit_operator
example_skip_dag
example_subdag_operator
example_subdag_operator.section-1
example_subdag_operator.section-2
example_trigger_controller_dag
example_trigger_target_dag
example_xcom
haha
latest_only
latest_only_with_trigger
test_utils
tutorial

SHOW:~ macbook$ airflow pause haha
[2018-09-07 21:35:41,366] {__init__.py:51} INFO - Using executor SequentialExecutor
[2018-09-07 21:35:41,496] {models.py:258} INFO - Filling up the DagBag from /Users/macbook/airflow/dags
Traceback (most recent call last):
File "/anaconda3/bin/airflow", line 32, in <module>
args.func(args)
File "/anaconda3/lib/python3.6/site-packages/airflow/utils/cli.py", line 74, in wrapper
return f(*args, **kwargs)
File "/anaconda3/lib/python3.6/site-packages/airflow/bin/cli.py", line 365, in pause
set_is_paused(True, args, dag)
File "/anaconda3/lib/python3.6/site-packages/airflow/bin/cli.py", line 379, in set_is_paused
dm.is_paused = is_paused
AttributeError: 'NoneType' object has no attribute 'is_paused'

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