遇到 let url = URL(string: str) 轉出來的url 結果是nil
解決方法：

var _url : String?
    var url : String {
        get {
            return _url ?? ""
        }
        set (newValue) {
            _url = newValue
            
            //個別網頁鏈接不能轉url
            //使用默認編碼（通常是 UTF-8）進行百分比編碼
            if let escapedString = newValue.addingPercentEncoding(withAllowedCharacters: .urlQueryAllowed) {
                let url = URL(string: escapedString)
                webView.load(URLRequest(url: url!))
            }
        }
    }