Flutter：仅当运行处于发布模式时才在空值上使用空检查运算符，而不是调试

Question

当我在调试模式下运行这个演示代码 (TabBarView) 时，一切正常。但是当我运行演示进入发布模式“Flutter 运行 --release”时，我得到了空值检查运算符。在我开始点击所有标签栏并点击第一个标签栏 return 之前，该应用程序无法运行。我试过 'flutter pub cache repair'，但没有用。（我用的是最新的flutter）

import 'package:flutter/material.dart';

void main() {
  WidgetsFlutterBinding.ensureInitialized();
  runApp(const MyApp());
}

class MyApp extends StatelessWidget {
  const MyApp({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return const MaterialApp(
      title: 'Tab Bar',
      home: MyTabBar(),
    );
  }
}

class MyTabBar extends StatefulWidget {
  const MyTabBar({Key? key}) : super(key: key);

  @override
  State<MyTabBar> createState() => _MyTabBarState();
}

class _MyTabBarState extends State<MyTabBar> with TickerProviderStateMixin {
  late TabController _controller;

  @override
  void initState() {
    _controller = TabController(length: 3, vsync: this);
    super.initState();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: const Text('Tab Bar'),
        bottom: TabBar(
          controller: _controller,
          tabs: const [
            Tab(text: 'Me', icon: Icon(Icons.menu_book_outlined)),
            Tab(text: 'What', icon: Icon(Icons.hourglass_bottom_rounded)),
            Tab(text: 'To', icon: Icon(Icons.room)),
          ],
        ),
      ),
      body: TabBarView(
        controller: _controller,
        children: const [
          Center(
            child: Text('Hello'),
          ),
          Center(
            child: Text('me'),
          ),
          Center(
            child: Text('again'),
          ),
        ],
      ),
    );
  }
}

这是我在控制台上得到的。

I/flutter ( 7521): I/flutter ( 7521): #0 I/flutter ( 7521): #1 I/flutter ( 7521): #2 I/flutter ( 7521): #3 I/flutter ( 7521): #4 I/flutter ( 7521): #5 I/flutter ( 7521): #6 I/flutter ( 7521): #7 I/flutter ( 7521): #8 I/flutter ( 7521): #9 I/flutter ( 7521): #10 I/flutter ( 7521): #11 I/flutter ( 7521): #12 I/flutter ( 7521): #13 I/flutter ( 7521): #14 I/flutter ( 7521): #15 I/flutter ( 7521): #16 I/flutter ( 7521): #17 I/flutter ( 7521): #18 I/flutter ( 7521): #19 I/flutter ( 7521): #20 I/flutter ( 7521): #21 I/flutter ( 7521): #22 I/flutter ( 7521): #23 I/flutter ( 7521): #24 I/flutter ( 7521): #25 I/flutter ( 7521): #26 I/flutter ( 7521): #27 I/flutter ( 7521): #28 I/flutter ( 7521): #29 I/flutter ( 7521): #30 I/flutter ( 7521): #31 I/flutter ( 7521): #32 I/flutter ( 7521): #33 I/flutter ( 7521): #34 I/flutter ( 7521): #35 I/flutter ( 7521): #36 I/flutter ( 7521): #37 I/flutter ( 7521): #38 I/flutter ( 7521): #39 I/flutter ( 7521): #40 I/flutter ( 7521): #41 I/flutter ( 7521): #42 I/flutter ( 7521): #43 I/flutter ( 7521): #44 I/flutter ( 7521): #45 I/flutter ( 7521): #46 I/flutter ( 7521): #47 I/flutter ( 7521): #48 I/flutter ( 7521): #49 I/flutter ( 7521): #50 I/flutter ( 7521): #51 I/flutter ( 7521): #52 I/flutter ( 7521): #53 I/flutter ( 7521): #54 I/flutter ( 7521): #55 I/flutter ( 7521): #56 I/flutter ( 7521): #57 I/flutter ( 7521): #58 I/flutter ( 7521): #59 I/flutter ( 7521): #60 I/flutter ( 7521): #61 I/flutter ( 7521): #62 I/flutter ( 7521): #63 I/flutter ( 7521): #64 I/flutter ( 7521): #65 I/flutter ( 7521): #66 I/flutter ( 7521): #67 I/flutter ( 7521): #68 I/flutter ( 7521): #69 I/flutter ( 7521): #70 I/flutter ( 7521): #71 I/flutter ( 7521): #72 I/flutter ( 7521): #73 I/flutter ( 7521): #74 I/flutter ( 7521): #75 I/flutter ( 7521): #76 I/flutter ( 7521): #77 I/flutter ( 7521): #78 I/flutter ( 7521): #79 I/flutter ( 7521): #80 I/flutter ( 7521): #81 I/flutter ( 7521): #82 I/flutter ( 7521): #83 I/flutter ( 7521): #84 I/flutter ( 7521): #85 I/flutter ( 7521): #86 I/flutter ( 7521): #87 I/flutter ( 7521): #88 I/flutter ( 7521): #89 I/flutter ( 7521): #90 I/flutter ( 7521): #91 I/flutter ( 7521): #92 I/flutter ( 7521): #93 I/flutter ( 7521): #97 I/flutter ( 7521): #98 I/flutter ( 7521): #99 I/flutter ( 7521): Null check operator used on a null value _PagePosition.applyViewportDimension (package:flutter/src/widgets/page_view.dart:455) RenderViewport.performLayout (package:flutter/src/rendering/viewport.dart:1456) RenderObject.layout (package:flutter/src/rendering/object.dart:1915) RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:116) RenderObject.layout (package:flutter/src/rendering/object.dart:1915) RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:116) RenderObject.layout (package:flutter/src/rendering/object.dart:1915) RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:116) RenderObject.layout (package:flutter/src/rendering/object.dart:1915) RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:116) RenderObject.layout (package:flutter/src/rendering/object.dart:1915) RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:116) RenderObject.layout (package:flutter/src/rendering/object.dart:1915) RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:116) RenderObject.layout (package:flutter/src/rendering/object.dart:1915) RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:116) RenderObject.layout (package:flutter/src/rendering/object.dart:1915) RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:116) RenderCustomPaint.performLayout (package:flutter/src/rendering/custom_paint.dart:545) RenderObject.layout (package:flutter/src/rendering/object.dart:1915) RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:116) RenderObject.layout (package:flutter/src/rendering/object.dart:1915) MultiChildLayoutDelegate.layoutChild (package:flutter/src/rendering/custom_layout.dart:171) _ScaffoldLayout.performLayout (package:flutter/src/material/scaffold.dart:1003) MultiChildLayoutDelegate._callPerformLayout (package:flutter/src/rendering/custom_layout.dart:240) RenderCustomMultiChildLayoutBox.performLayout (package:flutter/src/rendering/custom_layout.dart:403) RenderObject.layout (package:flutter/src/rendering/object.dart:1915) RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:116) RenderObject.layout (package:flutter/src/rendering/object.dart:1915) RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:116) _RenderCustomClip.performLayout (package:flutter/src/rendering/proxy_box.dart:1376) RenderObject.layout (package:flutter/src/rendering/object.dart:1915) RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:116) RenderObject.layout (package:flutter/src/rendering/object.dart:1915) RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:116) RenderObject.layout (package:flutter/src/rendering/object.dart:1915) RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:116) RenderObject.layout (package:flutter/src/rendering/object.dart:1915) RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:116) RenderObject.layout (package:flutter/src/rendering/object.dart:1915) RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:116) RenderObject.layout (package:flutter/src/rendering/object.dart:1915) RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:116) RenderObject.layout (package:flutter/src/rendering/object.dart:1915) RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:116) RenderObject.layout (package:flutter/src/rendering/object.dart:1915) RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:116) RenderObject.layout (package:flutter/src/rendering/object.dart:1915) RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:116) RenderObject.layout (package:flutter/src/rendering/object.dart:1915) RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:116) RenderObject.layout (package:flutter/src/rendering/object.dart:1915) RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:116) RenderObject.layout (package:flutter/src/rendering/object.dart:1915) RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:116) RenderObject.layout (package:flutter/src/rendering/object.dart:1915) RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:116) RenderObject.layout (package:flutter/src/rendering/object.dart:1915) RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:116) RenderObject.layout (package:flutter/src/rendering/object.dart:1915) RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:116) RenderObject.layout (package:flutter/src/rendering/object.dart:1915) RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:116) RenderObject.layout (package:flutter/src/rendering/object.dart:1915) RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:116) RenderOffstage.performLayout (package:flutter/src/rendering/proxy_box.dart:3462) RenderObject.layout (package:flutter/src/rendering/object.dart:1915) RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:116) RenderObject.layout (package:flutter/src/rendering/object.dart:1915) _RenderTheatre.performLayout (package:flutter/src/widgets/overlay.dart:749) RenderObject.layout (package:flutter/src/rendering/object.dart:1915) RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:116) RenderObject.layout (package:flutter/src/rendering/object.dart:1915) RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:116) RenderObject.layout (package:flutter/src/rendering/object.dart:1915) RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:116) RenderObject.layout (package:flutter/src/rendering/object.dart:1915) RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:116) RenderObject.layout (package:flutter/src/rendering/object.dart:1915) RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:116) RenderObject.layout (package:flutter/src/rendering/object.dart:1915) RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:116) RenderObject.layout (package:flutter/src/rendering/object.dart:1915) RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:116) RenderObject.layout (package:flutter/src/rendering/object.dart:1915) RenderView.performLayout (package:flutter/src/rendering/view.dart:165) RenderObject._layoutWithoutResize (package:flutter/src/rendering/object.dart:1757) PipelineOwner.flushLayout (package:flutter/src/rendering/object.dart:887) RendererBinding.drawFrame (package:flutter/src/rendering/binding.dart:504) WidgetsBinding.drawFrame (package:flutter/src/widgets/binding.dart:892) RendererBinding._handlePersistentFrameCallback (package:flutter/src/rendering/binding.dart:370) SchedulerBinding._invokeFrameCallback (package:flutter/src/scheduler/binding.dart:1146) SchedulerBinding.handleDrawFrame (package:flutter/src/scheduler/binding.dart:1083) SchedulerBinding._handleDrawFrame (package:flutter/src/scheduler/binding.dart:997) _invoke (dart:ui/hooks.dart:151) PlatformDispatcher._drawFrame (dart:ui/platform_dispatcher.dart:308) _drawFrame (dart:ui/hooks.dart:115) (elided 3 frames from dart:async)

Answer 1

您可以使用此代码：在您的代码中，您可以使用 sized box 来控制高度。它会显示其他错误可能是因为您没有处理控制器。

class NavigationScreen extends StatefulWidget {
  static String id = 'NavigationScreen';

  const NavigationScreen({Key? key}) : super(key: key);

  @override
  _NavigationScreenState createState() => _NavigationScreenState();
}

class _NavigationScreenState extends State<NavigationScreen> {
  int _selectedIndex = 0;
  final PageStorageBucket bucket = PageStorageBucket();
  List<Widget> _widgetOptions = [
    HomeScreen(
      key: PageStorageKey('Home'),
    ),
    CartScreen(key: PageStorageKey('Cart')),
    Profile(key: PageStorageKey('Profile'))
  ];

  @override
  void initState() {
    super.initState();
   }

  void _onItemTap(int index) {
    setState(() {
      _selectedIndex = index;
    });
  }

  Widget _bottomNavigationBar(int selectedIndex) => BottomNavigationBar(
        showSelectedLabels: false,
        showUnselectedLabels: false,
        backgroundColor: ThemeText.bottomNavBarColor,
        // fixedColor: Color(0xff707070),
        selectedItemColor: ThemeText.primaryColor,
        unselectedItemColor: ThemeText.bottomNavBarIconUnSelectedColor,
        items: [
          BottomNavigationBarItem(
              label: 'Home',
              icon: Icon(
                Icons.home_outlined,
              )),
          BottomNavigationBarItem(
              label: 'Cart', icon: Icon(Icons.shopping_cart_outlined)),
          BottomNavigationBarItem(
              label: 'Profile', icon: Icon(Icons.person_outlined)),
        ],
        currentIndex: _selectedIndex,
        onTap: _onItemTap,
      );

  @override
  Widget build(BuildContext context) {
    return Scaffold(
        // appBar: AppBar(
        //   title: Text('Home'),
        // ),
        // backgroundColor: Color(0xff707070),
        body: PageStorage(
          child: _widgetOptions[_selectedIndex],
          bucket: bucket,
        ),
        bottomNavigationBar: _bottomNavigationBar(_selectedIndex));
  }

}

Answer 2

看起来像是 Flutter 错误：https://github.com/flutter/flutter/issues/101007

Flutter 团队现在可以重现，因此修复可能就在眼前。有一个临时解决方法建议稍微延迟应用程序启动，但这显然不是解决方案，只是一个临时 hack。

Flutter：仅当 运行 处于发布模式时才在空值上使用空检查运算符，而不是调试

Flutter: Null check operator used on a null value Only when running in release mode, not debug

release

flutter

Flutter：仅当运行处于发布模式时才在空值上使用空检查运算符，而不是调试