[MDNS] Null safe and Android V2

[atrope]

Making the MDNS Plugin null safe(#57) and upgrade to V2 Embedding

[guyluz11]

Fix for #60

[guyluz11]

Can you please fix the example app as well to support Android V2?.
https://github.com/djthorpe/flutter/tree/master/mdns_plugin/example
I am getting this error with migration guide to android V2 embedding:

This app is using a deprecated version of the Android embedding.
To avoid unexpected runtime failures, or future build failures, try to migrate this app to the V2 embedding.
Take a look at the docs for migrating an app: https://github.com/flutter/flutter/wiki/Upgrading-pre-1.12-Android-projects
The plugin `mdns_plugin` requires your app to be migrated to the Android embedding v2. Follow the steps on https://flutter.dev/go/android-project-migration and re-run this command.

Plus I get null safety errors when using in new flutter app

E/flutter ( 8286): [ERROR:flutter/lib/ui/ui_dart_state.cc(209)] Unhandled Exception: Null check operator used on a null value
E/flutter ( 8286): #0      EventChannel.binaryMessenger (package:flutter/src/services/platform_channel.dart:473:86)
E/flutter ( 8286): #1      EventChannel.receiveBroadcastStream.<anonymous closure> (package:flutter/src/services/platform_channel.dart:493:7)
E/flutter ( 8286): #2      EventChannel.receiveBroadcastStream.<anonymous closure> (package:flutter/src/services/platform_channel.dart:492:64)
E/flutter ( 8286): #3      _runGuarded (dart:async/stream_controller.dart:773:24)
E/flutter ( 8286): #4      _BroadcastStreamController._subscribe (dart:async/broadcast_stream_controller.dart:207:7)
E/flutter ( 8286): #5      _ControllerStream._createSubscription (dart:async/stream_controller.dart:786:19)
E/flutter ( 8286): #6      _StreamImpl.listen (dart:async/stream_impl.dart:473:9)
E/flutter ( 8286): #7      new MDNSPlugin (package:mdns_plugin/mdns_plugin.dart:115:44)
E/flutter ( 8286): #8      main (package:app_flutter_test/main.dart:6:25)
E/flutter ( 8286): #9      _runMainZoned.<anonymous closure>.<anonymous closure> (dart:ui/hooks.dart:129:25)
E/flutter ( 8286): #10     _rootRun (dart:async/zone.dart:1428:13)
E/flutter ( 8286): #11     _CustomZone.run (dart:async/zone.dart:1328:19)
E/flutter ( 8286): #12     _runZoned (dart:async/zone.dart:1863:10)
E/flutter ( 8286): #13     runZonedGuarded (dart:async/zone.dart:1851:12)
E/flutter ( 8286): #14     _runMainZoned.<anonymous closure> (dart:ui/hooks.dart:125:5)
E/flutter ( 8286): #15     _delayEntrypointInvocation.<anonymous closure> (dart:isolate-patch/isolate_patch.dart:297:19)
E/flutter ( 8286): #16     _RawReceivePortImpl._handleMessage (dart:isolate-patch/isolate_patch.dart:192:12)
E/flutter ( 8286): 
E/flutter ( 8286): [ERROR:flutter/lib/ui/ui_dart_state.cc(209)] Unhandled Exception: Null check operator used on a null value
E/flutter ( 8286): #0      MethodChannel.binaryMessenger (package:flutter/src/services/platform_channel.dart:132:86)
E/flutter ( 8286): #1      MethodChannel._invokeMethod (package:flutter/src/services/platform_channel.dart:157:36)
E/flutter ( 8286): #2      MethodChannel.invokeMethod (package:flutter/src/services/platform_channel.dart:340:12)
E/flutter ( 8286): #3      MDNSPlugin.startDiscovery (package:mdns_plugin/mdns_plugin.dart:157:33)
E/flutter ( 8286): #4      main (package:app_flutter_test/main.dart:7:14)
E/flutter ( 8286): #5      _runMainZoned.<anonymous closure>.<anonymous closure> (dart:ui/hooks.dart:129:25)
E/flutter ( 8286): #6      _rootRun (dart:async/zone.dart:1428:13)
E/flutter ( 8286): #7      _CustomZone.run (dart:async/zone.dart:1328:19)
E/flutter ( 8286): #8      _runZoned (dart:async/zone.dart:1863:10)
E/flutter ( 8286): #9      runZonedGuarded (dart:async/zone.dart:1851:12)
E/flutter ( 8286): #10     _runMainZoned.<anonymous closure> (dart:ui/hooks.dart:125:5)
E/flutter ( 8286): #11     _delayEntrypointInvocation.<anonymous closure> (dart:isolate-patch/isolate_patch.dart:297:19)
E/flutter ( 8286): #12     _RawReceivePortImpl._handleMessage (dart:isolate-patch/isolate_patch.dart:192:12)
E/flutter ( 8286):