Flutter SnakeBar弹不出来，报Scaffold.of() called with a context that does not contain a Scaffold.错误

原因：当BuildContext在Scaffold之前时，调用Scaffold.of(context)会报错。这时可以通过Builder Widget来解决，代码如下：

  var _scaffoldkey = new GlobalKey();
  @override
  Widget build(BuildContext context) {
    return Scaffold(
        key: _scaffoldkey,）
  }
 void showSnackBar(String message) {
    var snackBar = SnackBar(content: Text(message));
    _scaffoldkey.currentState.showSnackBar(snackBar);
  }