Fix two null safety analysis issues and update goldens

[devoncarew]

Fix two null safety analysis issues.

List which issues are fixed by this PR. You must list at least one issue.

• these analysis issues are currently breaking the Dart SDK's flutter-analyze CI bot (https://ci.chromium.org/ui/p/dart/builders/ci.sandbox/flutter-analyze/15160/overview)

warning • The receiver can't be null, so the null-aware operator '?.' is unnecessary •
       lib/feature_discovery/feature_discovery.dart:224:34 • invalid_null_aware_operator
warning • The receiver can't be null, so the null-aware operator '?.' is unnecessary •
       lib/feature_discovery/feature_discovery.dart:240:36 • invalid_null_aware_operator

If you had to change anything in the flutter/tests repo, include a link to the migration guide as per the breaking change policy.

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read the Flutter Style Guide recently, and have followed its advice.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• All existing and new tests are passing.

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[guidezpl]

Integrated the changes from #597 to fix the test deadlock

[devoncarew]

Thanks for fixing the PR up can landing it!

I'm assuming it's only a smaller set of folks that can trigger the repo's checks?

[guidezpl]

I think so, but I don't have access to those settings. @godofredoc might know more

[copperyp]

Hi guidezpl

build & run fail :(

cd gallery
flutter config --enable-windows-desktop
flutter pub get
flutter run -d windows

lib/feature_discovery/feature_discovery.dart(224,35): error G0EB489DC: Method 'addPostFrameCallback' cannot be called on 'SchedulerBinding?' because it is p
otentially null. [D:\src\gallery\build\windows\flutter\flutter_assemble.vcxproj]
lib/feature_discovery/feature_discovery.dart(240,37): error G0EB489DC: Method 'addPostFrameCallback' cannot be called on 'SchedulerBinding?' because it is p
otentially null. [D:\src\gallery\build\windows\flutter\flutter_assemble.vcxproj]

env
[√] Flutter (Channel stable, 2.10.1, on Microsoft Windows [Version 10.0.19044.1526], locale zh-CN)
[√] Android toolchain - develop for Android devices (Android SDK version 31.0.0)
[√] Chrome - develop for the web
[√] Visual Studio - develop for Windows (Visual Studio Community 2019 16.11.9)
[√] Android Studio (version 4.2)
[√] IntelliJ IDEA Ultimate Edition (version 2019.1)
[√] Connected device (4 available)
[√] HTTP Host Availability

• No issues found!

i revert file [feature_discovery.dart] to previous was build & run success!

[guidezpl]

Please read the README