Support null-safety

[isorensen]

Hello! thanks for the awesome package.

Dart 2.12 and flutter 2.0.0 are now in stable with the null-safety feature.

Migrating this package to null-safety would help a lot. :)

[tenhobi]

Hi @isorensen, this project doesn't contain any code. I haven't checked, should I do something else? 🤔

[tenhobi]

Found it. https://dart.dev/null-safety/migration-guide It should be migrated. Thanks for the tip! 👍

[tenhobi]