airflow: Serialized DAG trigger via UI error - AttributeError: 'NoneType' object has no attribute 'create_dagrun'

Apache Airflow version: 1.10.10

Kubernetes version (if you are using kubernetes) (use kubectl version): -

Environment:

• Cloud provider or hardware configuration: MacBook Pro (2019), i5, 16gp RAM
• OS (e.g. from /etc/os-release): Debian GNU/Linux 9 (stretch)
• Kernel (e.g. uname -a): Linux 9d5d03cba741 4.19.76-linuxkit #1 SMP Thu Oct 17 19:31:58 UTC 2019 x86_64 GNU/Linux
• Install tools: Run Airflow in Docker containers
• Others: What happened: When use CeleryExecutor + Dag Serialization triggering DAGs via Web UI gets error:

Traceback (most recent call last):
  File "/usr/local/lib/python3.7/site-packages/flask/app.py", line 2447, in wsgi_app
    response = self.full_dispatch_request()
  File "/usr/local/lib/python3.7/site-packages/flask/app.py", line 1952, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "/usr/local/lib/python3.7/site-packages/flask/app.py", line 1821, in handle_user_exception
    reraise(exc_type, exc_value, tb)
  File "/usr/local/lib/python3.7/site-packages/flask/_compat.py", line 39, in reraise
    raise value
  File "/usr/local/lib/python3.7/site-packages/flask/app.py", line 1950, in full_dispatch_request
    rv = self.dispatch_request()
  File "/usr/local/lib/python3.7/site-packages/flask/app.py", line 1936, in dispatch_request
    return self.view_functions[rule.endpoint](**req.view_args)
  File "/usr/local/lib/python3.7/site-packages/airflow/www_rbac/decorators.py", line 121, in wrapper
    return f(self, *args, **kwargs)
  File "/usr/local/lib/python3.7/site-packages/flask_appbuilder/security/decorators.py", line 109, in wraps
    return f(self, *args, **kwargs)
  File "/usr/local/lib/python3.7/site-packages/airflow/www_rbac/decorators.py", line 56, in wrapper
    return f(*args, **kwargs)
  File "/usr/local/lib/python3.7/site-packages/airflow/utils/db.py", line 74, in wrapper
    return func(*args, **kwargs)
  File "/usr/local/lib/python3.7/site-packages/airflow/www_rbac/views.py", line 1050, in trigger
    external_trigger=True
  File "/usr/local/lib/python3.7/site-packages/airflow/utils/db.py", line 74, in wrapper
    return func(*args, **kwargs)
  File "/usr/local/lib/python3.7/site-packages/airflow/models/dag.py", line 1818, in create_dagrun
    return self.get_dag().create_dagrun(run_id=run_id,
AttributeError: 'NoneType' object has no attribute 'create_dagrun'

What you expected to happen: DAG can be triggered via Web UI without exception.

How to reproduce it:

1. Deploy Airflow Cluster via docker-compose (example below)
2. Using CeleryExecutor in separated Docker containers for webserver, scheduler, and worker
3. Disable mount DAG files on webserver container and configure DAG Serialisation:

AIRFLOW__CORE__STORE_SERIALIZED_DAGS=True
AIRFLOW__CORE__MIN_SERIALIZED_DAG_UPDATE_INTERVAL=30
AIRFLOW__CORE__STORE_DAG_CODE=True

1. Open Web UI and try to trigger any DAG.
2. Gets error AttributeError: 'NoneType' object has no attribute 'create_dagrun'

Anything else we need to know:

Used docker-compose.yml (see NOTE!! comment):

version: "3.7"

services:
  postgres:
    container_name: airflow_postgres
    image: postgres:10
    environment:
      POSTGRES_HOST: "postgres"
      POSTGRES_PORT: "5432"
      POSTGRES_DB: "airflow"
      POSTGRES_USER: "airflow"
      POSTGRES_PASSWORD: "airflow"

  redis:
    container_name: airflow_redis
    image: 'redis:5.0.5'

  webserver:
    container_name: airflow_webserver
    image: airflow-dev:latest
    depends_on:
      - postgres
      - redis
    env_file:
      .env
    ports:
      - "8080:8080"
    command: webserver
    # NOTE!! If uncomment mount bellow, trigger dag via web UI works.
    #volumes:
    #- ./dags:/usr/local/airflow/dags

  scheduler:
    container_name: airflow_scheduler
    image: airflow-dev:latest
    depends_on:
      - webserver
    env_file:
      .env
    command: scheduler
    volumes:
      - ./dags:/usr/local/airflow/dags

  flower:
    container_name: airflow_flower
    image: airflow-dev:latest
    restart: always
    depends_on:
      - redis
    env_file:
      .env
    environment:
        CELERY_CONFIG_MODULE: airflow.config_templates.default_celery
    ports:
        - 5555:5555
    command: flower

  worker:
    container_name: airflow_worker
    image: airflow-dev:latest
    restart: always
    ports: ['8793:8793']
    depends_on:
      - webserver
      - scheduler
    env_file:
      .env
    command: worker -q tech
    volumes:
      - ./dags:/usr/local/airflow/dags