我是新手，不会摆弄它。所以，请对我有耐心。

单击PopupMenuButton的特定菜单项时，将引发以下异常，但始终只能第二次：

'package:flutter/src/widgets/navigator.dart':失败断言：line 1846 pos 12:'！_debuglocked':不是true。

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

  final String title;
  final MaterialPageRoute pageRoute;
}

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,
),

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

  ...
}

正如您所看到的，有私有变量用于保存WordPair对象，但也用于菜单选择。

_popupMenus列表是在“构建重写”中设置的：

@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));
    }
    ...

在定义的PopupMenuChoice的MaterialPageRoute中，私有变量可能是access（例如_saved)。

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