Exception thrown when exiting the Reply study

[pennzht]

Describe the bug
When exiting the "Reply" study, an exception is thrown, but it is only visible on the console. See "To Reproduce" for more information.

Flutter doctor

Logs

Downloading android-arm-profile/darwin-x64 tools...                 1.5s
Downloading android-arm-release/darwin-x64 tools...                 1.2s
Downloading android-arm64-profile/darwin-x64 tools...               1.1s
Downloading android-arm64-release/darwin-x64 tools...               0.7s
Downloading android-x64-profile/darwin-x64 tools...                 1.2s
Downloading android-x64-release/darwin-x64 tools...                 1.0s
[✓] Flutter (Channel master, 1.22.0-10.0.pre.269, on Mac OS X 10.15.6 19G2021 x86_64, locale en-DE)
    • Flutter version 1.22.0-10.0.pre.269 at /Users/tianguang/Documents/f/flutter
    • Framework revision 51fa704642 (2 days ago), 2020-09-19 11:27:04 -0700
    • Engine revision 4b8477d115
    • Dart version 2.10.0 (build 2.10.0-142.0.dev)

 
[✓] Android toolchain - develop for Android devices (Android SDK version 29.0.3)
    • Android SDK at /Users/tianguang/Library/Android/sdk
    • Platform android-29, build-tools 29.0.3
    • Java binary at: /Applications/Android Studio with
      Blaze.app/Contents/jre/jdk/Contents/Home/bin/java
    • Java version OpenJDK Runtime Environment (build 1.8.0_242-release-1644-b3-6222593)
    • All Android licenses accepted.

[!] Xcode - develop for iOS and macOS (Xcode 11.2.1)
    • Xcode at /Applications/Xcode 11.2.1.app/Contents/Developer
    • Xcode 11.2.1, Build version 11B500
    ✗ CocoaPods installed but not working.
        You appear to have CocoaPods installed but it is not working.
        This can happen if the version of Ruby that CocoaPods was installed with is different from
        the one being used to invoke it.
        This can usually be fixed by re-installing CocoaPods. For more info, see
        https:/flutter/flutter/issues/14293.
      To re-install CocoaPods, run:
        sudo gem install cocoapods

[✓] Chrome - develop for the web
    • Chrome at /Applications/Google Chrome.app/Contents/MacOS/Google Chrome

[!] Android Studio (version 4.1)
    • Android Studio at /Applications/Android Studio with Blaze.app/Contents
    ✗ Flutter plugin not installed; this adds Flutter specific functionality.
    ✗ Dart plugin not installed; this adds Dart specific functionality.
    • Java version OpenJDK Runtime Environment (build 1.8.0_242-release-1644-b3-6222593)

[✓] Android Studio (version 3.5)
    • Android Studio at /Applications/Android Studio.app/Contents
    • Flutter plugin version 39.0.3
    • Dart plugin version 191.8423
    • Java version OpenJDK Runtime Environment (build 1.8.0_202-release-1483-b49-5587405)

[✓] Connected device (3 available)
    • macOS (desktop)  • macos      • darwin-x64     • Mac OS X 10.15.6 19G2021 x86_64
    • Web Server (web) • web-server • web-javascript • Flutter Tools
    • Chrome (web)     • chrome     • web-javascript • Google Chrome 85.0.4183.102

! Doctor found issues in 2 categories.

To Reproduce

1. Use Flutter master or dev.
2. In the Gallery's root directory, run flutter run -d chrome.
3. When the app loads, open the "Reply" study.
4. After a few seconds, click "Back" to return to home screen.
5. Read the logs.

The following logs are produced on master. A similar log is produced on dev.

Logs

Launching lib/main.dart on Chrome in debug mode...
Syncing files to device Chrome...                                       
18,832ms (!)                                       
Debug service listening on ws://127.0.0.1:60622/lQyX_N22b3Y=

Warning: Flutter's support for web development is not stable yet and hasn't
been thoroughly tested in production environments.
For more information see https://flutter.dev/web

🔥  To hot restart changes while running, press "r" or "R".
For a more detailed help message, press "h". To quit, press "q".
══╡ EXCEPTION CAUGHT BY SCHEDULER LIBRARY ╞═════════════════════════════════════════════════════════
The following assertion was thrown during a scheduler callback:
There are multiple heroes that share the same tag within a subtree.
Within each subtree for which heroes are to be animated (i.e. a PageRoute subtree), each Hero must
have a unique non-null tag.
In this case, multiple heroes had the following tag: <default FloatingActionButton tag>
├# Here is the subtree for one of the offending heroes: Hero

When the exception was thrown, this was the stack:
dart-sdk/lib/_internal/js_dev_runtime/private/ddc_runtime/errors.dart 216:49  throw_
packages/flutter/src/widgets/heroes.dart 266:11                               <fn>
packages/flutter/src/widgets/heroes.dart 276:9                                inviteHero
packages/flutter/src/widgets/heroes.dart 296:21                               visitor
packages/flutter/src/widgets/framework.dart 6117:14                           visitChildren
packages/flutter/src/widgets/heroes.dart 309:14                               visitor
packages/flutter/src/widgets/framework.dart 4695:14                           visitChildren
packages/flutter/src/widgets/heroes.dart 309:14                               visitor
packages/flutter/src/widgets/framework.dart 6117:14                           visitChildren
packages/flutter/src/widgets/heroes.dart 309:14                               visitor
packages/flutter/src/widgets/framework.dart 6117:14                           visitChildren
packages/flutter/src/widgets/heroes.dart 309:14                               visitor
packages/flutter/src/widgets/framework.dart 6117:14                           visitChildren
packages/flutter/src/widgets/heroes.dart 309:14                               visitor
packages/flutter/src/widgets/framework.dart 6230:16                           visitChildren
packages/flutter/src/widgets/heroes.dart 309:14                               visitor
packages/flutter/src/widgets/framework.dart 4695:14                           visitChildren
packages/flutter/src/widgets/heroes.dart 309:14                               visitor
packages/flutter/src/widgets/framework.dart 4695:14                           visitChildren
packages/flutter/src/widgets/heroes.dart 309:14                               visitor
packages/flutter/src/widgets/framework.dart 4695:14                           visitChildren
packages/flutter/src/widgets/heroes.dart 309:14                               visitor
packages/flutter/src/widgets/framework.dart 4695:14                           visitChildren
packages/flutter/src/widgets/heroes.dart 309:14                               visitor
packages/flutter/src/widgets/framework.dart 6117:14                           visitChildren
packages/flutter/src/widgets/heroes.dart 309:14                               visitor
packages/flutter/src/widgets/framework.dart 4695:14                           visitChildren
packages/flutter/src/widgets/heroes.dart 309:14                               visitor
packages/flutter/src/widgets/framework.dart 6117:14                           visitChildren
packages/flutter/src/widgets/framework.dart 3260:5                            visitChildElements
packages/flutter/src/widgets/heroes.dart 312:12                               _allHeroesFor
packages/flutter/src/widgets/heroes.dart 839:53                               [_startHeroTransition]
packages/flutter/src/widgets/heroes.dart 814:11                               <fn>
packages/flutter/src/scheduler/binding.dart 1117:15                           [_invokeFrameCallback]
packages/flutter/src/scheduler/binding.dart 1063:9                            handleDrawFrame
packages/flutter/src/scheduler/binding.dart 971:5                             [_handleDrawFrame]
lib/_engine/engine/window.dart 800:13                                         _invoke
lib/_engine/engine/window.dart 396:5                                          invokeOnDrawFrame
lib/_engine/engine.dart 228:18                                                <fn>
════════════════════════════════════════════════════════════════════════════════════════════════════

Expected Behavior
Able to enter and exit the "Reply" app without raising any errors.

Screenshots
(None)

Desktop (please complete the following information):

• OS: macOS Catalina 10.15.6
• Browser: Chrome 85.0.4183.102

Additional context
(None)