iOS 工程 制作私有pod 包含 flutter build 产物. 在setup 时 crash, 没有plugin

```
FlutterBoost.instance().setup(application, delegate: router) { engine in
assert(FlutterBoost.instance().plugin() != nil, "must not be nil")
completion?()
}
```

## Steps to Reproduce
**A small application to reproduce the bug(最小化可复现的demo)**

1. ...
2. ...
3. ...

**Flutter Boost Version**
**Target Platform:**
**Target OS version/browser:**
**Devices:**

## Logs
add your crash log or something else.

```
```

Reproduce the crash when an iOS private pod contains Flutter build artifacts, using the FlutterBoost.instance().setup call shown in the report. Inspect why FlutterBoost.instance().plugin() is nil during the setup callback; done means setup completes without the assertion or crash and the plugin is available.

Written by the indexing model from the issue text.