Crash when using the share plugin

@noborder My app using the [share plugin](https://pub.dev/packages/share) and crash happen when User tapped on the close button of UIActivityController

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

1. Present a FLBFlutterViewContainer A
2. Tap to share button in FLBFlutterViewContainer A
3. The App presents a UIActivityController B
4. Tap to close button on UIActivityController B -> app crash

**Flutter Boost Version**
1.12.13+hotfix.9
**Target Platform:**
iOS
**Target OS version/browser:**
13.1.2
**Devices:**
iPhone

Start with the minimal reproduction flow from FLBFlutterViewContainer A through UIActivityController B, focusing on the close-button dismissal on iOS 13.1.2. Confirm the crash, then trace the Flutter Boost iOS integration involved in presenting and dismissing the controller; done means closing the share UI no longer crashes the app.

Written by the indexing model from the issue text.