颤动：使用navigator.dart时失败的断言

我是Flutter的新手并且玩弄它 . 所以，请耐心等待我 .

单击PopupMenuButton的特定菜单项时会抛出以下异常，但始终只是第二次：

'package：flutter / src / widgets / navigator.dart'：断言失败：第1846行第12行：'！_debugLocked'：不是真的 .

这里设置：

为了指定菜单项，已经定义了以下类：

class PopupMenuChoice {
  const PopupMenuChoice({this.title, this.pageRoute});

  final String title;
  final MaterialPageRoute pageRoute;
}

在AppBar的actions属性中定义PopupMenuButton：

new PopupMenuButton<PopupMenuChoice>(
    itemBuilder: (BuildContext context) {
      return _popupMenus.map((PopupMenuChoice choice) {
        return new PopupMenuItem<PopupMenuChoice>(
          value: choice,
          child: new Text(choice.title),
        );
      }).toList();
    },
    onSelected: _popupMenuSelected,
),

相应的小部件在下面的类中定义（AppBar在此类的“return new Scaffold”中创建）：

class RandomWordsState extends State<RandomWords> {
  final _suggestions = <WordPair>[];
  final _saved = new Set<WordPair>();
  final _popupMenus = <PopupMenuChoice>[];

  ...
}

正如您所看到的，有用于保存WordPair对象的私有变量，但也有用于菜单选项的私有变量 .

_popupMenus列表在“构建覆盖”中设置：

@override
Widget build(BuildContext context) {
    // Setup page routes
    if (_popupMenus.where((p) => p.title == 'Saved Suggestions').length == 0) {
      final _pageRouteSavedSuggestions = new MaterialPageRoute(
        builder: (context) {
          final tiles = _saved.map(
            (pair) {
              return new ListTile(
                title: new Text(
                  pair.asPascalCase,
                  style: _biggerFont,
                ),
              );
            },
          );
          final divided = ListTile
              .divideTiles(
                context: context,
                tiles: tiles,
              )
              .toList();

          return new Scaffold(
            appBar: new AppBar(
              title: new Text('Saved Suggestions'),
            ),
            body: new ListView(children: divided),
          );
        },
      );
      _popupMenus.add(new PopupMenuChoice(
          title: 'Saved Suggestions', pageRoute: _pageRouteSavedSuggestions));
    }

    if (_popupMenus.where((p) => p.title == 'TEST Page').length == 0) {
      final _pageRouteTest = new MaterialPageRoute(
        builder: (context) {
          return new Scaffold(
            appBar: new AppBar(
              title: new Text('TEST Page'),
            ),
            body: new Text('Some content...'),
          );
        },
      );
      _popupMenus.add(
          new PopupMenuChoice(title: 'TEST Page', pageRoute: _pageRouteTest));
    }
    ...

在PopupMenuChoice的已定义的MaterialPageRoute中，私有变量可能是访问权限（例如_saved） .

这里是onSelected的PopupMenuButton的相应事件处理程序：

void _popupMenuSelected(PopupMenuChoice choice) {
  Navigator.of(context).push(choice.pageRoute);
}

任何人都可以解释为什么抛出这个异常？它怎么能被阻止？

谢谢，罗杰

在特定菜单项上单击第二次时来自调试控制台的其他信息：

E / flutter（17133）：[错误：topaz / lib / tonic / logging / dart_error.cc（16）]未处理的异常：E / flutter（17133）：'package：flutter / src / widgets / routes.dart'：断言失败：第177行pos 12：'！_ transnsitionCompleter.isCompleted'：处理后无法安装MaterialPageRoute . E / flutter（17133）：＃0 _AssertionError._doThrowNew（dart：core / runtime / liberrors_patch.dart:37:39）E / flutter（17133）：＃1 _AssertionError._throwNew（dart：core / runtime / liberrors_patch.dart： 33：5）E / flutter（17133）：＃2 TransitionRoute.install（包：flutter / src / widgets / routes.dart）E / flutter（17133）：＃3 ModalRoute.install（包：flutter / src / widgets / routes.dart：740：11）E / flutter（17133）：＃4 NavigatorState.push（package：flutter / src / widgets / navigator.dart:1444：11）E / flutter（17133）：＃5 RandomWordsState.build . _popupMenuSelected（file：/// D：/Flutter%20Projects/startup_namer/lib/main.dart:166：29）E / flutter（17133）：＃6 _PopupMenuButtonState.showButtonMenu . （包：flutter / src / material / popup_menu.dart）E / flutter（17133）：＃7 _RootZone.runUnary（dart：async / zone.dart:1381:54）E / flutter（17133）：＃8 _FutureListener.handleValue （dart：async / future_impl.dart:129:18）E / flutter（17133）：＃9 Future._propagateToListeners.handleValueCallback（dart：async / future_impl.dart:633:45）E / flutter（17133）：＃10 Future ._propagateToListeners（dart：async / future_impl.dart:662:32）E / flutter（17133）：＃11 Future._completeWithValue（dart：async / future_impl.dart:477：5）E / flutter（17133）：＃12 Future ._asyncComplete . （dart：async / future_impl.dart:507：7）E / flutter（17133）：＃13 _microtaskLoop（dart：async / schedule_microtask.dart:41:21）E / flutter（17133）：＃14 _startMicrotaskLoop（dart：async /schedule_microtask.dart:50:5）