User status updates come from the server in a map where keys are user IDs.
If user.trackStatus() is called for a user model without an ID, the model
cannot identify its status updates and silently misses them. It's quite hard to
notice that a user rendered in the UI doesn't receive live status updates.
Also, it's not immediately obvious what's the reason of the problem.
A warning will be very helpful here.
9932f144c2