Skip NextBus update if integration is still loading (#123564)

* Skip NextBus update if integration is still loading

Fixes a race between the loading thread and
update thread leading to an unrecoverable error

* Use async_at_started

* Use local copy of _route_stops to avoid NextBus race condition

* Update homeassistant/components/nextbus/coordinator.py

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Co-authored-by: Joost Lekkerkerker <joostlek@outlook.com>