尝试使用queryCursors（Firestore）获取更多文档时出现问题

Question

我正在尝试使用查询游标加载我的Firestore数据库，以使其看起来像无限滚动。当我查询第一个批处理时一切正常，但是当我滚动我的应用程序时崩溃并给我这个错误：

  

致命错误：在隐式展开可选值时意外发现nil

我知道这意味着什么，但是我找不到正确的方法。错误是从此行开始的：

queryRef = db.collection("Ingredients").start(afterDocument: lastDocument).limit(to: 2)

这是我的代码：

var fetchMoreIngredients = false
    var reachEnd = false
    let leadingScreensForBatching: CGFloat = 3.0

    var ingredients = [Ingredients]()
    var lastDocument:DocumentSnapshot!

func loadData(completion: @escaping(_ ingredients:[Ingredients])->()){

        let db = Firestore.firestore()
        let lastIngredient = self.ingredients.last
        var queryRef: Query!

        if lastIngredient == nil{
          queryRef = db.collection("Ingredients").limit(to: 2)
        }else{
            queryRef = db.collection("Ingredients").start(afterDocument: lastDocument).limit(to: 2)
        }

        queryRef.getDocuments(){
            querySnapshot, error in
            if let error = error {
                print("\(error.localizedDescription)")
            }else if let querySnapshot = querySnapshot {
                if (querySnapshot.isEmpty == false){

                    let res = querySnapshot.documents.compactMap({Ingredients(dictionary: $0.data())})
                    self.ingredients.append(contentsOf: res)

                }
            }
            return completion(self.ingredients)
        }
    }

func scrollViewDidScroll(_ scrollView: UIScrollView) {

        let off = scrollView.contentOffset.y
        let off1 = scrollView.contentSize.height

        if off > off1 - scrollView.frame.height{
            if !fetchMoreIngredients && !reachEnd{
                beginBatchFetch()
            }
        }

    }

    func beginBatchFetch(){

        fetchMoreIngredients = true

        loadData{ newIngredients in
            self.ingredients.append(contentsOf: newIngredients)
            self.reachEnd = newIngredients.count == 0
            self.fetchMoreIngredients = false
            self.tableView.reloadData()

        }
    }