Question: Your code works even though you do not initialize celery backend

[VinayKadalagi]

I ran your code locally and it works well.
I build similar system of long running task and capturing custom states. But in my case it fails with this error

https://stackoverflow.com/questions/31852219/why-asyncresult-returns-nonetype-after-task-is-done

And it is resolved after i initialized celery object with backend as below,

celery = Celery(app.name, broker=app.config['CELERY_BROKER_URL'], backend=app.config['CELERY_RESULT_BACKEND'])

Please let me know how it is working in your case.??

[miguelgrinberg]

The result backend is in the Flask config dictionary, which then is applied to the Celery instance with:

celery.conf.update(app.config)

Have you done that?