Swift iOS Firebase -If the FirebaseAuth.FIRUser Object isn't nil how is it possible for the .email on it to return nil?

I use Firebase's Email/Password Sign-In Method to create an account for a user. Using that method the user must have an email address to get authenticated into Firebase.

FirebaseAuth has a FIRUser object named User

When a user first creates an account or logs into an existing account in the callback the User object gets initialized:

Create Account:

Auth.auth().createUser(withEmail: emailTextField.text!, password: passwordTextField.text!, completion: {
            (user, error) in

            // user's auth credentials are now created but user can still be nil

Logging into existing account:

Auth.auth().signIn(withEmail: self.emailTextField.text!, password: self.passwordTextField.text!, completion: {
            (user, error) in

            // user's auth credentials are now accessed but user can still be nil

The User object has an .email Optional of type String on it that contain's the user's email address.

I use a singleton class to manage everything that happens through Firebase and in both situations above when the User objects gets initialized I pass that data through to my Firebase singleton's class properties. I know the User object can come back as nil so I run an if-let to make sure it isn't.

My question is if using the Email/Password Sign-In Method if the User object isn't nil in the callback is it possible that the .email can be nil even though it's necessary to have it to make an account or log in?

After I run the if-let statements I use guard statements to check and see if the .email isn't nil but it seems like it either can run before the the FirebaseSingleton properties gets initialized (which means it will be nil) or it might be unnecessary if the User object is guaranteed to return it with a value.

Btw in both situations wether the user is creating and account or logging in the emailTextFiled and passwordTextField will not be nil. I run whitespace, .isEmpty, and != nil checks on them.

My Code:

class FirebaseSingleton{

    static let sharedInstance = FirebaseSingleton()

    var dbRef: DatabaseReference? = Database.database().reference()
    var storageDbRef: StorageReference? = Storage.storage().reference(forURL: "gs://blablabla.appspot.com")
    var currentUser: User? = Auth.auth().currentUser
    var currentUserID: String? = Auth.auth().currentUser?.uid
    var currentUserEmail: String? = Auth.auth().currentUser?.email
    var currentUserPhotoUrl: URL? = Auth.auth().currentUser?.photoURL
    var currentUserDisplayName: String? = Auth.auth().currentUser?.displayName
}

Creating an Account:

Auth.auth().createUser(withEmail: emailTextField.text!, password: passwordTextField.text!, completion: {
                (user, error) in

                if error != nil { return }

                if let user = user{

                    let sharedInstance = FirebaseSingleton.sharedInstance
                    sharedInstance.currentUser = user
                    sharedInstance.currentUserID = user.uid
                    sharedInstance.currentUserEmail = user.email // at this point is it possible for this to be nil?
                    sharedInstance.currentUserPhotoUrl = user.photoURL
                    sharedInstance.currentUserDisplayName = user.displayName
                }else{
                   return
                }

                // is this guard statement necessary?
                guard let email = user.uid else { return }
})

Logging into an existing account:

Auth.auth().signIn(withEmail: emailTextField.text!, password: passwordTextField.text!, completion: {
                (user, error) in

                if error != nil { return }

                if let user = user{

                    let sharedInstance = FirebaseSingleton.sharedInstance
                    sharedInstance.currentUser = user
                    sharedInstance.currentUserID = user.uid
                    sharedInstance.currentUserEmail = user.email // at this point is it possible for this to be nil?
                    sharedInstance.currentUserPhotoUrl = user.photoURL
                    sharedInstance.currentUserDisplayName = user.displayName
                }else{
                   return
                }

                // is this guard statements necessary?    
                guard let email = user.uid else { return }
})