Make library null-safe

[tonyborchert]

As this package is not null safe, I went ahead and made the PR #11 to make the library null-safe (and therefore compatible with older versions), but also modernise the package (e.g. replace @required with required and also use TextButton instead of the deprecated FlatButton)

[ninest]

Thank you for taking the time and effort to make these changes. I am unfortunately quite busy and do not have time to work on Flutter or this project. For now, I'll suggest making the repository with your changes the source of the package. I'm sorry about this.

I'll leave this PR open for when I have more time in the future

[tonyborchert]

Did not take long, just wanted to help on this awesome library :) , take your time you've done enough, at the moment I can't even update to null-safety, as the other libraries im depending on, are also still in an old version, so it doesn't really matter.

[aidanshilling]

Hi is this pull request going to be merged soon? I would love to use this library.