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

将插件添加到 pubspec.yaml，并将 README 中提到的条目添加到 Info.plist 后，在一个干净的 Flutter 应用中复现该问题。从 withLocalUrl 设置为 true 的 WebViewScaffold 开始，检查 iOS 上的崩溃行为；当应用在此设置下不再崩溃时即表示完成，如相关则检查 Android。

由索引模型根据 Issue 内容生成。