Null check operator used on a null value in Isolate

Hello and thanks for the package !

I try to compress the video in an isolate and i have the following error when making a Top-Level function :

```
I/flutter (20324): ----------------FIREBASE CRASHLYTICS----------------
I/flutter (20324): Exception: Null check operator used on a null value
I/flutter (20324):
I/flutter (20324): #0 MethodChannel.binaryMessenger (package:flutter/src/services/platform_channel.dart:121:86)
I/flutter (20324): #1 MethodChannel.setMethodCallHandler (package:flutter/src/services/platform_channel.dart:375:5)
I/flutter (20324): #2 CompressMixin.initProcessCallback (package:video_compress/src/progress_callback/compress_mixin.dart:11:14)
I/flutter (20324): #3 new _VideoCompressImpl._ (package:video_compress/src/video_compress/video_compressor.dart:15:5)
I/flutter (20324): #4 _VideoCompressImpl.instance (package:video_compress/src/video_compress/video_compressor.dart:21:45)
I/flutter (20324): #5 VideoCompress (package:video_compress/src/video_compress/video_compressor.dart:30:56)
I/flutter (20324): #6 computeCompression (package:myapppackage/main/api.dart:24:10)
I/flutter (20324): #7 _IsolateConfiguration.apply (package:flutter/src/foundation/_isolates_io.dart:84:34)
I/flutter (20324): #8 _spawn. (package:flutter/src/foundation/_isolates_io.dart:91:65)
I/flutter (20324): #9 _spawn. (package:flutter/src/foundation/_isolates_io.dart:90:5)
I/flutter (20324): #10 Timeline.timeSync (dart:developer/timelin
I/flutter (20324): ----------------------------------------------------
```