- Each property is checked with `OWSAssertDebug` in the initializer.
They are also initialized in a simple way when the app is launching,
so any that are `nil` would be noticed quickly.
- Getting the shared environment also checks it with `OWSAssertDebug`.
In addition, it’s currently an implicitly-unwrapped optional in Swift,
so it’d already be crashing if it’s unexpectedly nil.