Retrieves a Firebase app instance.
When called with no arguments, the default app is returned. When an app name is provided, the app corresponding to that name is returned.
An exception is thrown if the app being retrieved has not yet been initialized.
Optional name: string
Optional name of the app to return. If no name is provided, the default is
The app corresponding to the provided app name. If no app name is provided, the default app is returned.