Plugin throws exceptions with flutter v1.10.7 beta

[Jon-Salmon]

On the latest beta version of flutter, dart errors are thrown when a purchase is requested.

I believe this may be due to this dart change with how Future<Null> is interpreted:
dart-lang/sdk#37985

E/flutter (13113): [ERROR:flutter/lib/ui/ui_dart_state.cc(148)] Unhandled Exception: type 'String' is not a subtype of type 'Null'
E/flutter (13113): #0      MethodChannel.invokeMethod (package:flutter/src/services/platform_channel.dart:321:13)
E/flutter (13113): <asynchronous suspension>
E/flutter (13113): #1      FlutterInappPurchase.requestPurchase (package:flutter_inapp_purchase/flutter_inapp_purchase.dart:229:29)
E/flutter (13113): <asynchronous suspension>

[hyochan]

Do you think you can help out giving any other PR?

[Jon-Salmon]

What was the motivation for some of the methods to return Future<Null> instead of Future? I believe this is the only change that is required.

[Jon-Salmon]

I have created pull request #117 which I believe fixes this issue.

Please test it however as I have only performed basic tests.