Migrate to NNBD

[SirusCodes]

Migrating this package to null-safety would be really great.
https://medium.com/dartlang/preparing-the-dart-and-flutter-ecosystem-for-null-safety-e550ce72c010

[Frezyx]

Hello @SirusCodes !
Thank you for this issue 👍🏻
I will check pr from @prasadsunny1 soon.

[Frezyx]

Pr #5 was merged.
This changes will be available in next package update on pub.dev.

[SirusCodes]

Congratulations @prasadsunny1 🎉