Back gesture shows a white screen on android instead of quitting the app #17

Describe the bug
Since 0.0.1-dev.10, there is a regression when using the back gesture (Android 11) on the main screen. A back should close the app, but since dev.10 it shows a white screen instead.

To Reproduce
Steps to reproduce the behavior:

1. Compile the https://github.com/FredJul/Warmd app and change flow_builder version to use the last one (currently 0.0.1-dev.11)
2. Press next on all questionnaire screen until you reached the score screen
3. Use the back gesture, it displays a white screen instead of quitting the app

Expected behavior
Same behavior than 0.0.1-dev.9, the back gesture on the score screen should quit the app

**Logs **
flutter doctor -v
[✓] Flutter (Channel stable, 1.22.5, on macOS 11.0.1 20B29 darwin-x64, locale fr-FR)
• Flutter version 1.22.5 at /Users/f.julian/Documents/flutter
• Framework revision 7891006299 (27 hours ago), 2020-12-10 11:54:40 -0800
• Engine revision ae90085a84
• Dart version 2.10.4

[✓] Android toolchain - develop for Android devices (Android SDK version 30.0.2)
• Android SDK at /Users/f.julian/Library/Android/sdk
• Platform android-30, build-tools 30.0.2
• ANDROID_HOME = /Users/f.julian/Library/Android/sdk
• Java binary at: /Applications/Android Studio.app/Contents/jre/jdk/Contents/Home/bin/java
• Java version OpenJDK Runtime Environment (build 1.8.0_242-release-1644-b3-6915495)
• All Android licenses accepted.

[✓] VS Code (version 1.52.0)
• VS Code at /Applications/Visual Studio Code.app/Contents
• Flutter extension version 3.17.0

Additional context
I believe #14 to be the culprit of this behavior change.

Also, https://github.com/FredJul/Warmd is not a big project (FlowBuilder is exclusively used in the main.dart file), but if you have difficulties reproducing this issue, I could try to do a smaller sample.