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firebase childByAutoId.key is optional

I m working with Firebase and Swift. I want to use the Firebase Function childByAutoId.key but it is not working. What I m doing wrong? The .key is optional... Normally it has to be non optional! Thanks for help!

import Foundation
import FirebaseDatabase

class AddWatchlistAPI {


    var REF_WATCHLIST = Database.database().reference().child("watchlists")


    static var shared: AddWatchlistAPI = AddWatchlistAPI()
    private init() {
    }

    //add a watchlist to the user
    func addWatchlistToDatabase(watchlistName: String, onSuccess: @escaping () -> Void) {

        let watchlistRef = REF_WATCHLIST
        let watchlistId = watchlistRef.childByAutoId().key //.key is optional ???? --> Normally it isn t optional

        let newWatchlistRef = watchlistRef.child(watchlistId)//here the error is warning

    }

}

Try as follows

1- let watchlistId:String = (watchlistRef.childByAutoId().key)!

or

2- let watchlistId:String = watchlistRef.childByAutoId().key ?? "" let watchlistId:String = watchlistRef.childByAutoId().key ?? ""

or

3-

var ref: DatabaseReference!

ref = Database.database().reference()

let llave = self.ref.child(name_of_child).childByAutoId().key

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