Flutter：基础库捕获到异常。查找已停用的小部件的祖先是不安全的

Question

我在具有Bloc状态管理的TabBarView内有一个PageView。第一个选项卡具有一个PageView，该页面绑定到bloc中的列表，当屏幕打开时，当我滚动到该PageView的另一个页面时，没有任何警报，但是当我切换到另一个选项卡并返回到具有PageView的第一个选项卡时，当我滚动到PageView的另一个Page时，控制台会在此消息下面发出多次警报。

Flutter Doctor一切正常。该应用程序除了显示这些消息外没有崩溃，这些消息是在我滑动页面时发生的，但是仅在切换选项卡并返回到第一个选项卡之后。

════════基础库捕获到异常Exception ══════════════════ 查找停用的小部件的祖先是不安全的。

import 'package:flutter/material.dart';
import '../bloc.dart';
import '../models.dart';

class Bookshelf extends StatefulWidget {
  @override
  _BookshelfState createState() => _BookshelfState();
}

class _BookshelfState extends State<Bookshelf>
    with TickerProviderStateMixin, AutomaticKeepAliveClientMixin {
  AnimationController _controller;
  Animation<double> _animation;
  PageController _pageController;
  final GlobalKey<ScaffoldState> scaffoldKey2 = new GlobalKey<ScaffoldState>();

  @override
  bool get wantKeepAlive => true;

  @override
  void initState() {
    super.initState();
    _controller = AnimationController(
      duration: Duration(milliseconds: 300),
      lowerBound: 0.0,
      upperBound: 1.0,
      vsync: this,
    );

    _animation = CurvedAnimation(
      parent: _controller,
      curve: Curves.easeIn,
    );

    _pageController = PageController(viewportFraction: 0.9);
  }

  @override
  Widget build(BuildContext context) {
    BlocProvider.of(context).hasOnboarded.listen((hasOnboarded) {
      if (hasOnboarded)
        _controller.forward();
      else
        _controller.reverse();
    });

    /// scroll position
    _pageController.addListener(() {
      BlocProvider.of(context).setScrollPosition(
        _pageController.offset / _pageController.position.maxScrollExtent,
      );
    });

    return Container(
      alignment: Alignment.bottomRight,
      child: SizeTransition(
        sizeFactor: _animation,
        axis: Axis.horizontal,
        axisAlignment: -1.0,
        child: Container(
          alignment: Alignment.bottomCenter,
          child: Container(
            padding: EdgeInsets.only(top: 48.0, bottom: 28.0),
            height: MediaQuery.of(context).size.height * .72,
            child: TabBarView(children: [
              StreamBuilder(
                stream: BlocProvider.of(context).books,
                initialData: <Book>[],
                builder: (context, AsyncSnapshot<List<Book>> snapshot) {
                  /// update on scroll

                  _pageController.addListener(() {
                    if (snapshot.data.isNotEmpty)
                      BlocProvider.of(context).setColor(
                        ColorTransition(
                          colors:
                              snapshot.data.map((book) => book.color).toList(),
                          offset: _pageController.offset,
                          maxExtent: _pageController.position.maxScrollExtent,
                        ),
                      );
                  });

                  return PageView(
                    controller: _pageController,
                    children: snapshot.data
                        .map((book) => MyBook(book: book))
                        .toList(),
                  );
                },
              ),
              Container(
                child: Center(
                  child: Text('primeiro'),
                ),
              ),
              Container(
                child: Center(
                  child: Text('segundo'),
                ),
              ),
            ]),
          ),
        ),
      ),
    );
  }

  @override
  void dispose() {
    // TODO: implement dispose
    super.dispose();
    _pageController.dispose();
    _controller.dispose();
  }
}

class MyBook extends StatelessWidget {
  final Book book;
  MyBook({this.book});

  @override
  Widget build(BuildContext context) {
    return Container(
      margin: EdgeInsets.symmetric(vertical: 24.0, horizontal: 10.0),
      decoration: BoxDecoration(
        color: Colors.grey[100],
        borderRadius: BorderRadius.circular(12.0),
        boxShadow: <BoxShadow>[
          BoxShadow(
            color: Colors.grey[600],
            blurRadius: 12.0,
            spreadRadius: -2.0,
            offset: Offset(0.0, 5.0),
          )
        ],
      ),
      child: ClipRRect(
        borderRadius: BorderRadius.circular(12.0),
        child: Column(
          crossAxisAlignment: CrossAxisAlignment.start,
          children: <Widget>[
            SizedBox(height: 10.0),
            Center(
              child: Image(
                image: book.hero,
                height: 180,
                fit: BoxFit.fitHeight,
              ),
            ),
            SizedBox(height: 12.0),
            Container(
              padding: EdgeInsets.symmetric(horizontal: 20.0),
              child: Column(
                crossAxisAlignment: CrossAxisAlignment.start,
                children: <Widget>[
                  Text(
                    book.title,
                    style:
                        TextStyle(fontSize: 24.0, fontWeight: FontWeight.w500),
                  ),
                  SizedBox(height: 18.0),
                  Text(
                    'Por ${book.author}',
                    style:
                        TextStyle(fontSize: 12.0, fontWeight: FontWeight.w500),
                  ),
                  SizedBox(height: 18.0),
                  Text(
                    'O livro apresenta uma dinamica muito importante do direito de empresas.',
                    style:
                        TextStyle(fontSize: 14.0, fontWeight: FontWeight.w500),
                  ),
                  SizedBox(height: 18.0),
                  Center(
                    child: RaisedButton(
                      padding: EdgeInsets.symmetric(
                          vertical: 14.0, horizontal: 20.0),
                      onPressed: () => print("read ${book.title}"),
                      color: book.color,
                      child: Text(
                        "LER O LIVRO",
                        style: TextStyle(color: Colors.white),
                      ),
                      shape: RoundedRectangleBorder(
                        borderRadius: BorderRadius.circular(99.9),
                      ),
                    ),
                  ),
                ],
              ),
            )
          ],
        ),
      ),
    );
  }
}