AppRegistryNotReady-Error when having contrib.postgres in INSTALLED_APPS

[Jann Haber]

After an upgrade to Django 2.2a1, I found the following problem:

$ python manage.py help
/home/test/.virtualenvs/testenv/lib/python3.7/site-packages/psycopg2/__init__.py:144: UserWarning: The psycopg2 wheel package will be renamed from release 2.8; in order to keep installing from binary please use "pip install psycopg2-binary" instead. For details see: <http://initd.org/psycopg/docs/install.html#binary-install-from-pypi>.
  """)
Traceback (most recent call last):
  File "manage.py", line 21, in <module>
    main()
  File "manage.py", line 17, in main
    execute_from_command_line(sys.argv)
  File "/home/test/.virtualenvs/testenv/lib/python3.7/site-packages/django/core/management/__init__.py", line 381, in execute_from_command_line
    utility.execute()
  File "/home/test/.virtualenvs/testenv/lib/python3.7/site-packages/django/core/management/__init__.py", line 357, in execute
    django.setup()
  File "/home/test/.virtualenvs/testenv/lib/python3.7/site-packages/django/__init__.py", line 24, in setup
    apps.populate(settings.INSTALLED_APPS)
  File "/home/test/.virtualenvs/testenv/lib/python3.7/site-packages/django/apps/registry.py", line 91, in populate
    app_config = AppConfig.create(entry)
  File "/home/test/.virtualenvs/testenv/lib/python3.7/site-packages/django/apps/config.py", line 116, in create
    mod = import_module(mod_path)
  File "/usr/lib/python3.7/importlib/__init__.py", line 127, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "<frozen importlib._bootstrap>", line 1006, in _gcd_import
  File "<frozen importlib._bootstrap>", line 983, in _find_and_load
  File "<frozen importlib._bootstrap>", line 967, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 677, in _load_unlocked
  File "<frozen importlib._bootstrap_external>", line 728, in exec_module
  File "<frozen importlib._bootstrap>", line 219, in _call_with_frames_removed
  File "/home/test/.virtualenvs/testenv/lib/python3.7/site-packages/django/contrib/postgres/apps.py", line 8, in <module>
    from django.db.migrations.writer import MigrationWriter
  File "/home/test/.virtualenvs/testenv/lib/python3.7/site-packages/django/db/migrations/writer.py", line 10, in <module>
    from django.db.migrations.loader import MigrationLoader
  File "/home/test/.virtualenvs/testenv/lib/python3.7/site-packages/django/db/migrations/loader.py", line 8, in <module>
    from django.db.migrations.recorder import MigrationRecorder
  File "/home/test/.virtualenvs/testenv/lib/python3.7/site-packages/django/db/migrations/recorder.py", line 9, in <module>
    class MigrationRecorder:
  File "/home/test/.virtualenvs/testenv/lib/python3.7/site-packages/django/db/migrations/recorder.py", line 22, in MigrationRecorder
    class Migration(models.Model):
  File "/home/test/.virtualenvs/testenv/lib/python3.7/site-packages/django/db/models/base.py", line 99, in __new__
    app_config = apps.get_containing_app_config(module)
  File "/home/test/.virtualenvs/testenv/lib/python3.7/site-packages/django/apps/registry.py", line 252, in get_containing_app_config
    self.check_apps_ready()
  File "/home/test/.virtualenvs/testenv/lib/python3.7/site-packages/django/apps/registry.py", line 135, in check_apps_ready
    raise AppRegistryNotReady("Apps aren't loaded yet.")
django.core.exceptions.AppRegistryNotReady: Apps aren't loaded yet.

To reproduce (running python 3.7.2):
Make a new python3 venv (python3 -m venv) and activate. Install django 2.2a1 and psycopg2 using pip. Use django-admin.py startproject to get a fresh installation of django. Add contrib.postgres to INSTALLED_APPS. Run the command above, which should display the help messages. The same thing happens also with any other manage.py command.

Note: The same thing is working with no problem when running Django 2.1.5