WebUI returns an error when logs that do not use a DAG list `None` as the DAG ID

[psigen]

Apache Airflow version: 2.0.0

Kubernetes version (if you are using kubernetes) (use kubectl version): N/A

Environment:

• Cloud provider or hardware configuration: docker-compose
• OS (e.g. from /etc/os-release): Docker apache/airflow sha256:b4f957bef5a54ca0d781ae1431d8485f125f0b5d18f3bc7e0416c46e617db265
• Kernel (e.g. uname -a): Linux c697ae3a0397 5.4.0-58-generic Hive Metastore Browser plugin #64~18.04.1-Ubuntu SMP Wed Dec 9 17:11:11 UTC 2020 x86_64 GNU/Linux
• Install tools: docker
• Others:

What happened:

When an event that does not include a DAG is logged in the UI, this event lists the DAG ID as "None". This "None" is treated as an actual DAG ID with a link, which throws an error if clicked.

Something bad has happened.
Please consider letting us know by creating a bug report using GitHub.

Python version: 3.6.12
Airflow version: 2.0.0
Node: 9097c882a712
-------------------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/airflow/.local/lib/python3.6/site-packages/flask/app.py", line 2447, in wsgi_app
    response = self.full_dispatch_request()
  File "/home/airflow/.local/lib/python3.6/site-packages/flask/app.py", line 1952, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "/home/airflow/.local/lib/python3.6/site-packages/flask/app.py", line 1821, in handle_user_exception
    reraise(exc_type, exc_value, tb)
  File "/home/airflow/.local/lib/python3.6/site-packages/flask/_compat.py", line 39, in reraise
    raise value
  File "/home/airflow/.local/lib/python3.6/site-packages/flask/app.py", line 1950, in full_dispatch_request
    rv = self.dispatch_request()
  File "/home/airflow/.local/lib/python3.6/site-packages/flask/app.py", line 1936, in dispatch_request
    return self.view_functions[rule.endpoint](**req.view_args)
  File "/home/airflow/.local/lib/python3.6/site-packages/airflow/www/auth.py", line 34, in decorated
    return func(*args, **kwargs)
  File "/home/airflow/.local/lib/python3.6/site-packages/airflow/www/decorators.py", line 97, in view_func
    return f(*args, **kwargs)
  File "/home/airflow/.local/lib/python3.6/site-packages/airflow/www/decorators.py", line 60, in wrapper
    return f(*args, **kwargs)
  File "/home/airflow/.local/lib/python3.6/site-packages/airflow/utils/session.py", line 65, in wrapper
    return func(*args, session=session, **kwargs)
  File "/home/airflow/.local/lib/python3.6/site-packages/airflow/www/views.py", line 2028, in graph
    dag = current_app.dag_bag.get_dag(dag_id)
  File "/home/airflow/.local/lib/python3.6/site-packages/airflow/utils/session.py", line 65, in wrapper
    return func(*args, session=session, **kwargs)
  File "/home/airflow/.local/lib/python3.6/site-packages/airflow/models/dagbag.py", line 171, in get_dag
    self._add_dag_from_db(dag_id=dag_id, session=session)
  File "/home/airflow/.local/lib/python3.6/site-packages/airflow/models/dagbag.py", line 227, in _add_dag_from_db
    raise SerializedDagNotFound(f"DAG '{dag_id}' not found in serialized_dag table")
airflow.exceptions.SerializedDagNotFound: DAG 'None' not found in serialized_dag table

What you expected to happen:

I expected None to not be a link or have it link to some sort of error page. Instead it throws an error.

How to reproduce it:

Run a CLI command such as airflow dags list, then go to /log/list/ in the web UI, and click on the None Dag Id for the logged event for the command.

Anything else we need to know:

This problem appears to occur every time.

[boring-cyborg]

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