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
"""Reads all DAGs in serialized_dag table.
        :param session: ORM Session
        :returns: a dict of DAGs read from database
        """
        serialized_dags = session.query(cls)
        dags = {}
        for row in serialized_dags:
            log.debug("Deserializing DAG: %s", row.dag_id)
            dag = row.dag
            
            if dag.dag_id == row.dag_id:
                dags[row.dag_id] = dag
            else:
                log.warning(
                    "dag_id Mismatch in DB: Row with dag_id '%s' has Serialised DAG with '%s' dag_id",
                    row.dag_id,
                    dag.dag_id,
                )
        return dags
    @property
    def dag(self):