Calling endpoint dags/{dag_id}/dagRuns for removed DAG returns "500 Internal Server Error" instead of "404 Not Found"

[lukasz-worek]

Apache Airflow version

Other Airflow 2 version (please specify below)

What happened

Apache Airflow version: 2.4.0

I remove DAG from storage then trigger it:

curl -X POST 'http://localhost:8080/api/dags/<DAG_ID>/dag_runs' --header 'Content-Type: application/json' --data '{"dag_run_id":"my_id"}'

it returns:

Traceback (most recent call last):
  File "/home/airflow/.local/lib/python3.8/site-packages/flask/app.py", line 2525, in wsgi_app
    response = self.full_dispatch_request()
  File "/home/airflow/.local/lib/python3.8/site-packages/flask/app.py", line 1822, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "/home/airflow/.local/lib/python3.8/site-packages/flask/app.py", line 1820, in full_dispatch_request
    rv = self.dispatch_request()
  File "/home/airflow/.local/lib/python3.8/site-packages/flask/app.py", line 1796, in dispatch_request
    return self.ensure_sync(self.view_functions[rule.endpoint])(**view_args)
  File "/home/airflow/.local/lib/python3.8/site-packages/connexion/decorators/decorator.py", line 68, in wrapper
    response = function(request)
  File "/home/airflow/.local/lib/python3.8/site-packages/connexion/decorators/uri_parsing.py", line 149, in wrapper
    response = function(request)
  File "/home/airflow/.local/lib/python3.8/site-packages/connexion/decorators/validation.py", line 196, in wrapper
    response = function(request)
  File "/home/airflow/.local/lib/python3.8/site-packages/connexion/decorators/validation.py", line 399, in wrapper
    return function(request)
  File "/home/airflow/.local/lib/python3.8/site-packages/connexion/decorators/response.py", line 112, in wrapper
    response = function(request)
  File "/home/airflow/.local/lib/python3.8/site-packages/connexion/decorators/parameter.py", line 120, in wrapper
    return function(**kwargs)
  File "/home/airflow/.local/lib/python3.8/site-packages/airflow/api_connexion/security.py", line 51, in decorated
    return func(*args, **kwargs)
  File "/home/airflow/.local/lib/python3.8/site-packages/airflow/utils/session.py", line 75, in wrapper
    return func(*args, session=session, **kwargs)
  File "/home/airflow/.local/lib/python3.8/site-packages/airflow/api_connexion/endpoints/dag_run_endpoint.py", line 310, in post_dag_run
    dag_run = dag.create_dagrun(
AttributeError: 'NoneType' object has no attribute 'create_dagrun'

What you think should happen instead

should response with 404 "A specified resource is not found."

How to reproduce

• remove existing DAG file from storage
• create a new DAG run using API endpoint /api/dags/<DAG_ID>/dag_runs for that deleted DAG

Operating System

18.04.1 Ubuntu

Versions of Apache Airflow Providers

No response

Deployment

Official Apache Airflow Helm Chart

Deployment details

No response

Anything else

No response

Are you willing to submit PR?

• Yes I am willing to submit a PR!

Code of Conduct

• I agree to follow this project's Code of Conduct

[boring-cyborg]

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[SamWheating]

I have replicated the issue locally in breeze and I'm working on a fix 👍