[BUG] App icon generation with flavors seems to corrupt project.pbxproj

### :information_source: Info

Version: `0.13.1`

### :speech_balloon: Description

Using the flutter_launcher_icons package I'm trying to create App icons for two flavors. I've created a flutter_launcher_icons-production.yaml and a flutter_launcher_icons-staging.yaml. My two flavors are named production and staging.

After running the command: `flutter pub run flutter_launcher_icons` I receive an error when opening the Xcode project, and get the following error when running the app again:


When I reset the project.pbxproj to the version before the modifications after the icon generations it works, and I can manually select the icons. It would however be great if this can be automatically done.

### :scroll: Pubspec.yaml

flutter_launcher_icons-production.yaml

```
flutter_launcher_icons:
android: true
adaptive_icon_background: "assets/launcher/production/android/background.png"
adaptive_icon_foreground: "assets/launcher/production/android/foreground.png"
ios: true
remove_alpha_ios: true
image_path: "assets/launcher/production/icon.png"
```

flutter_launcher_icons-staging.yaml

```
flutter_launcher_icons:
android: true
adaptive_icon_background: "assets/launcher/staging/android/background.png"
adaptive_icon_foreground: "assets/launcher/staging/android/foreground.png"
ios: true
remove_alpha_ios: true
image_path: "assets/launcher/staging/icon.png"
```