How to manage user once signed in? Facebook, firebase, post feeds, IOS

Trying to manage the signed in account, by assigning it to metadata (a username and image). I've successfully signed in the account with facebook, now i just need firebase to accept the metadata associated with that user, so that I have a reference to make social media posts with.

The problem is I have no idea what I'm doing, I've followed a few tutorials on how to login with firebase/facebook and I'm trying to combine the code from each tutorial.

Once signed in, I run a function called getUserData(), which I believe is supposed to generate an array of user image URL's that can be referenced in firebase database (to retrieve later).

The app crashes on this line: self.currentUserImageUrl = postDict["userImg"] as! String

The error I get is: "Thread 1: Fatal error: Unexpectedly found nil while unwrapping an Optional value."

I really wish someone could teach me what this problem means so that i can move forward.

This logs the facebook user in:

@objc func handleCustomFBLogin(){

FBSDKLoginManager().logIn(withReadPermissions: ["email", "public_profile"], from: self){
(result, err) in
if err != nil {
print("Custom FB login failed:", err)
return

}

self.showEmailAdress()
}

}

I think this provides a firebase reference for the facebook user:

func showEmailAdress() {

let accessToken = FBSDKAccessToken.current()
guard let accessTokenString = accessToken?.tokenString else
{return}

    let credential = FacebookAuthProvider.credential(withAccessToken:    FBSDKAccessToken.current().tokenString)

    Auth.auth().signIn(with: credential, completion: { (user,
    error ) in
    if error != nil {
    print("Something went wrong with the FB user:", error)
    return
    }
    self.setupUser(userUid: (user?.uid)!)
    KeychainWrapper.standard.set((user?.uid)!, forKey: "uid")
    self.performSegue(withIdentifier: "toFeed", sender: nil)
    print("Successfully logged in with user:", error)
    })
    FBSDKGraphRequest(graphPath: "/me", parameters: ["fields": "id, name,      email"]).start { (connection, result, err) in

            if err != nil{
                print("Failed to start graph request", err ?? "")
                return
            }
            print(result ?? "")
        }
    }

I think this handles the metadata for the user:

func setupUser(userUid: String) {
if let imageData = UIImageJPEGRepresentation(self.userImgView.image!, 0.2) {
let imgUid = NSUUID().uuidString
let metaData = StorageMetadata()
Storage.storage().reference().child(imgUid).putData(imageData, metadata: metaData) { (metadata, error) in

let downloadURL = metadata?.downloadURL()?.absoluteString

let userData = [
"username": self.usernameField.text!,
"userImg": downloadURL
] as [String : Any]

Database.database().reference().child("users").child(userUid).setValue(userData)
self.performSegue(withIdentifier: "toFeed", sender: nil)
}
}
}

App crashes on this function:

func getUsersData(){
let uid = KeychainWrapper.standard.string(forKey: "uid")
Database.database().reference().child("users").observeSingleEvent(of: .value) { (snapshot) in
if let postDict = snapshot.value as? [String : AnyObject] {
self.currentUserImageUrl = postDict["userImg"] as! String 
self.tableView.reloadData()
}
}

}