Null safety

[andre-paraense]

Requirements

• [x] I have added test coverage for new or changed functionality
• [x] I have followed the repository's pull request submission guidelines
• [x] I have validated my changes against all supported platform versions

Related issues

Lib code was not compatible with null safety.

Describe the solution you've provided

Migrated to null safety

How to test?

Run flutter test. Alternatively, run the example app.