cancelCompression doesn't work on iOS (v2.1.1)

I made a small demo of the issue (see below). Issue is related to iOS platform

If you don't cancel the process everything seems to be working just fine.


But if you stop the process (using `VideoCompress.cancelCompression` method) and immediately try to start compression again (same video or another, doesn't matter) you will get error:

```
flutter: Bad state: VideoCompress Error:
Method: compressVideo
Already have a compression process, you need to wait for the process to finish or stop it
```


At the same time if you stop the process (`VideoCompress.cancelCompression `) and wait enough for video compression to complete you will see that starting new compression works as expected.

Source code: [video_compress_bug.zip](https://github.com/jonataslaw/VideoCompress/files/5714540/video_compress_bug.zip)

Everything related to this bug happens in `viewer.dart` file