Happens every time 


Python version: 3.8.10
Airflow version: 2.1.0
Node: airflow-web-55974db849-5bdxq
-------------------------------------------------------------------------------
Traceback (most recent call last):
  File "/opt/bitnami/airflow/venv/lib/python3.8/site-packages/flask/app.py", line 2447, in wsgi_app
    response = self.full_dispatch_request()
  File "/opt/bitnami/airflow/venv/lib/python3.8/site-packages/flask/app.py", line 1952, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "/opt/bitnami/airflow/venv/lib/python3.8/site-packages/flask/app.py", line 1821, in handle_user_exception
    reraise(exc_type, exc_value, tb)
  File "/opt/bitnami/airflow/venv/lib/python3.8/site-packages/flask/_compat.py", line 39, in reraise
    raise value
  File "/opt/bitnami/airflow/venv/lib/python3.8/site-packages/flask/app.py", line 1950, in full_dispatch_request
    rv = self.dispatch_request()
  File "/opt/bitnami/airflow/venv/lib/python3.8/site-packages/flask/app.py", line 1936, in dispatch_request
    return self.view_functions[rule.endpoint](**req.view_args)
  File "/opt/bitnami/airflow/venv/lib/python3.8/site-packages/airflow/www/auth.py", line 34, in decorated
    return func(*args, **kwargs)
  File "/opt/bitnami/airflow/venv/lib/python3.8/site-packages/airflow/www/decorators.py", line 97, in view_func
    return f(*args, **kwargs)
  File "/opt/bitnami/airflow/venv/lib/python3.8/site-packages/airflow/www/decorators.py", line 60, in wrapper
    return f(*args, **kwargs)
  File "/opt/bitnami/airflow/venv/lib/python3.8/site-packages/airflow/www/views.py", line 4004, in list
    self._calculate_graph()
  File "/opt/bitnami/airflow/venv/lib/python3.8/site-packages/airflow/www/views.py", line 4023, in _calculate_graph
    for dag, dependencies in SerializedDagModel.get_dag_dependencies().items():
  File "/opt/bitnami/airflow/venv/lib/python3.8/site-packages/airflow/utils/session.py", line 70, in wrapper
    return func(*args, session=session, **kwargs)
  File "/opt/bitnami/airflow/venv/lib/python3.8/site-packages/airflow/models/serialized_dag.py", line 321, in get_dag_dependencies
    dependencies[row[0]] = [DagDependency(**d) for d in row[1]]
TypeError: 'NoneType' object is not iterable




tests/models/test_serialized_dag.py
"""Unit tests for SerializedDagModel."""
    def setUp(self):
        clear_db_serialized_dags()
    def tearDown(self):
        clear_db_serialized_dags()
    def test_dag_fileloc_hash(self):
        """Verifies the correctness of hashing file path."""
        assert DagCode.dag_fileloc_hash('/airflow/dags/test_dag.py') == 33826252060516589
    def _write_example_dags(self):
        example_dags = make_example_dags(example_dags_module)
        for dag in example_dags.values():
            SDM.write_dag(dag)
        return example_dags
    def test_write_dag(self):
        """DAGs can be written into database."""
        example_dags = self._write_example_dags()
        with create_session() as session:
            for dag in example_dags.values():
                assert SDM.has_dag(dag.dag_id)
                result = session.query(SDM.fileloc, SDM.data).filter(SDM.dag_id == dag.dag_id).one()
                assert result.fileloc == dag.full_filepath
                
                SerializedDAG.validate_schema(result.data)
    def test_serialized_dag_is_updated_only_if_dag_is_changed(self):