Adapt to use sound null check

**Is your feature request related to a problem? Please describe.**
Feature - new versions of dart requires a sound null type checking.

**Describe the solution you'd like**
Follow the guide described at [https://dart.dev/null-safety/unsound-null-safety](https://dart.dev/null-safety/unsound-null-safety) to improve and add sound null checks.

**Additional context**
As of Dart SDK version: 2.18.1 it's no longer possible to use this library without sound null checks.