Sometimes request.session.session_key is None

According to John's suggestion.

I fixed the problem by this snippet:

if not request.session.session_key:
    request.session.save()
session_id = request.session.session_key

As per documentation:

SessionStore.create() is designed to create a new session (i.e. one not loaded from the session store and with session_key=None). save() is designed to save an existing session (i.e. one loaded from the session store). Calling save() on a new session may also work but has a small chance of generating a session_key that collides with an existing one. create() calls save() and loops until an unused session_key is generated.

Means it is safer to use create() instead of save(). So you can try like this:

if not request.session.session_key:
    request.session.create()
session_id = request.session.session_key