iOS运行后后黑屏

## Steps to Reproduce
按照null-safety中example示例集成的iOS，运行后黑屏，帮忙看一下是什么问题，谢谢您了
[repo](https://github.com/realkimmy/FlutterBoostDemo)

**Flutter Boost Version** v3.0-null-safety-preview.11
**Target Platform:** ios
**Target OS version/browser:** ios15
**Devices:** iphone13

## Logs
add your crash log or something else.

```
```

Use the linked FlutterBoostDemo repository and its null-safety example as the entry point. Reproduce on iOS 15 with an iPhone 13, then collect the missing crash logs and `flutter doctor -v` output while comparing the integration with Flutter Boost v3.0-null-safety-preview.11. Done means the iOS app no longer opens to a black screen and the cause is documented.

Written by the indexing model from the issue text.