withLocalUrl() let's whole Flutter-App crash on iOS

## System info

Issue occurs on: iOS maybe on Android too
Plugin version: 0.3.8
Flutter doctor output:

```
[✓] Flutter (Channel stable, v1.9.1+hotfix.2, on Mac OS X 10.14.6 18G87, locale de-DE)

[✓] Android toolchain - develop for Android devices (Android SDK version 28.0.3)
[✓] Xcode - develop for iOS and macOS (Xcode 10.3)
[✓] Android Studio (version 3.5)
[✓] IntelliJ IDEA Ultimate Edition (version 2018.2.4)
[✓] Connected device (1 available)

• No issues found!
```

You can easily reproduce use a clean flutter app with flutter create. After that setup the plugin by adding it to pubspec.yaml and go to Info.plist and make the entry mentioned in the readme. In your code use WebViewScaffold and the the withLocalUrl to true --> App crashes

Reproduce the issue in a clean Flutter app after adding the plugin to pubspec.yaml and the README-mentioned entry in Info.plist. Start from WebViewScaffold with withLocalUrl set to true and inspect the iOS crash behavior; done means the app no longer crashes in this setup, with Android checked if relevant.

Scritto dal modello di indicizzazione a partire dal testo della issue.