#116 changed Future<Null> to Future due to dart sdk change

[Jon-Salmon]

I believe this is the only change required for compatibility with Flutter v1.10 and higher.

Please check that this doesn't break anything I have missed, I have only tested with a single in-app-purchase and run the tests.

[hyochan]

Thanks for the fixes. If I remember correctly, the Future method required generic type to be passed and the Null was safer than the void (correct me if I am wrong).

Let's have a go with this fix and see how others use it.

[Jon-Salmon]

I believe up until now Future<Null> would be handled as if it was Future<dynamic> but this has now changed.

I guess the best solution from a readability point of view would be to use Future<dynamic> but this wouldn't effect the code running as it is identical to just using Future