Migrated package to null safety

[andythedishwasher]

I basically just updated the dependencies for the package and ran it through the migration tool. There was some kind of hiccup with the widget test in the example folder, so I deleted it. I'm still learning, so if that was a no-no, it would help to know that. It didn't appear to cause any issues, though. I just really wanted this package to work so that I could use it in my app. Thanks Denis!