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




airflow/models/serialized_dag.py
"""Deletes a DAG with given dag_id.
        :param dag_id: dag_id to be deleted
        :param session: ORM Session
        """
        
        session.execute(cls.__table__.delete().where(cls.dag_id == dag_id))
    @classmethod
    @provide_session
    def remove_deleted_dags(cls, alive_dag_filelocs: List[str], session=None):
        """Deletes DAGs not included in alive_dag_filelocs.
        :param alive_dag_filelocs: file paths of alive DAGs
        :param session: ORM Session
        """
        alive_fileloc_hashes = [DagCode.dag_fileloc_hash(fileloc) for fileloc in alive_dag_filelocs]
        log.debug(
            "Deleting Serialized DAGs (for which DAG files are deleted) from %s table ", cls.__tablename__
        )
        
        session.execute(
            cls.__table__.delete().where(
                and_(cls.fileloc_hash.notin_(alive_fileloc_hashes), cls.fileloc.notin_(alive_dag_filelocs))
            )
        )
    @classmethod
    @provide_session
    def has_dag(cls, dag_id: str, session: Session = None) -> bool: