Before using Firebase SDK, we must run the code
FirebaseApp.configure()
The documentation says it will throw an error if something fails, but it returns nil, and I don't know how can I know exactly when it will be done.
From the documentation of configure:
Configures the default Firebase app with the provided options. The default app is named __FIRAPP_DEFAULT. Raises an exception if any configuration step fails. This method is thread safe.
It says that "This method is thread safe and contains synchronous file I/O", which means its job will be finished before the next line executes, as opposed to something that's asynchronous, like UIViewController.present(_:animated:completion:), which might not complete presenting the VC before the next line executes.