更新后-展开一个可选值时意外找到nil

Question

更新项目后，出现此错误：

  

在展开可选值时意外发现nil

class Search {
    private var _username: String!
    private var _userImg: String!
    private var _userKey: String!
    private var _userRef: DatabaseReference!

    var currentUser = KeychainWrapper.standard.string(forKey: "uid")

    var username: String {
        return _username <- error
    }

    var userImg: String {
        return _userImg
    }

    var userKey: String{
        return _userKey
    }

    init(username: String, userImg: String) {
        _username = username
        _userImg = userImg
    }

    init(userKey: String, postData: Dictionary<String, AnyObject>) {
        _userKey = userKey

        if let username = postData["username"] as? String {
            _username = username
        }

        if let userImg = postData["userImg"] as? String {
            _userImg = userImg
        }

        _userRef = Database.database().reference().child("messages").child(_userKey)
    }
}

它在Swift 3和Firebase 3.0.2下运行良好，但是现在，所有内容都已更新，它一直崩溃。这并不是以前其他任何问题的重复。