Harden bootstrap context registrations when apps are missing

It's not expected that an app would be unavailable when the app
container is created but when services are registered, but Sentry tells
me on Nextcloud 21 there is an edge case where this can happen.
Therefore this patch hardens the code a bit to log a meaningful error
message and skipping the next code instead of logging a php notice for
the undefined index and an exception for calling a method on null.

Signed-off-by: Christoph Wurst <christoph@winzerhof-wurst.at>