"Select Widget Mode" button disabled after hot restart on flutter web

## Steps to Reproduce

- Run a flutter web app
- Open flutter inspector pane
- The "Select Widget Mode" button is enabled and works
- Edit something and hot-restart
- The "Select Widget Mode" is disabled

## Version info

> [✓] Flutter (Channel beta, v1.17.0, on Linux, locale fr_FR.UTF-8)
> • Flutter version 1.17.0 at /home/william/Documents/Otera/flutter
> • Framework revision d3ed9ec945 (il y a 8 jours), 2020-04-06 14:07:34 -0700
> • Engine revision c9506cb8e9
> • Dart version 2.8.0 (build 2.8.0-dev.18.0 eea9717938)
>
>
> [✓] Android toolchain - develop for Android devices (Android SDK version 28.0.3)
> • Android SDK at /home/william/Android/Sdk
> • Platform android-29, build-tools 28.0.3
> • Java binary at: /opt/android-studio/jre/bin/java
> • Java version OpenJDK Runtime Environment (build 1.8.0_212-release-1586-b4-5784211)
> • All Android licenses accepted.
>
> [✓] Chrome - develop for the web
> • Chrome at google-chrome
>
> [✓] Android Studio (version 3.6)
> • Android Studio at /opt/android-studio
> • Flutter plugin version 45.1.1
> • Dart plugin version 192.7761
> • Java version OpenJDK Runtime Environment (build 1.8.0_212-release-1586-b4-5784211)
>
> [✓] VS Code (version 1.44.0)
> • VS Code at /usr/share/code
> • Flutter extension version 3.9.1
>
> [✓] Connected device (2 available)
> • Chrome • chrome • web-javascript • Google Chrome 81.0.4044.92
> • Web Server • web-server • web-javascript • Flutter Tools
>
> • No issues found!