The registry has only a STAGING model, so load_production returns None — and this server 'starts' anyway, then explodes with TypeError on the first request when predict indexes into None. Stage 1's rule: startup fails LOUDLY if nothing is in production. Fix it to check MODEL right after loading and print "FATAL: no production model for 'churn' - refusing to start" instead of serving.
The break is on line 14 — but read the whole snippet first.