Swift Firebase completion handler not working - swift

Question

My code calls Firebase and gets user information and user images. Once that's loaded, the app populates an image field and a text label. At least, that's what's supposed to happen. But instead, I'm getting fatal error: Index out of range error. What am I missing?
Here's my code:
#IBOutlet weak var userImage: UIImageView!
#IBOutlet weak var instructionsLabel: UILabel!
var currentUserName: String!
func loadExistingUsers(completion: #escaping () -> ()) {
ref.child("members").observe(.childAdded) { (snapshot: FIRDataSnapshot) in
if let dict = snapshot.value as? [String : Any] {
let userPhotoUrl = dict["profileImageUrl"] as! String
let userFirstName = dict["firstName"] as! String
let userBirthday = dict["birthday"] as! Int
let userPasscode = dict["passcode"] as! Int
let userGender = dict["gender"] as! String
let isUserChildOrParent = dict["childParent"] as! String
let storageRef = FIRStorage.storage().reference(forURL: userPhotoUrl)
storageRef.data(withMaxSize: 1 * 1024 * 1024, completion: { (data, error) in
let pic = UIImage(data: data!)
let user = User(profilePhoto: pic!,
userFirstName: userFirstName,
userBirthday: userBirthday,
userPasscode: userPasscode,
userGender: userGender,
isUserChildOrParent: isUserChildOrParent)
self.users.append(user)
self.users.sort(by: {$0.birthday < $1.birthday})
})
completion()
}
}
}
and my ViewDidLoad call:
loadExistingUsers {
self.currentUserName = self.users[0].firstName // error here
self.instructionsLabel.text = "Choose daily and weekly job assignments for \(self.users[0].firstName)."
self.userImage.image = self.users[0].photo
}
What am I doing wrong? I know my base code is good because I've tested it. Is it just the completion handler that's the problem? Any help would be greatly appreciated.