Migrate to Sound Null Safety

[tsvillain]

Sound null safety is now the default for new projects (from Flutter 2.2), adding protection against null reference exceptions and enabling developers to express non-nullable types in their code.

📝Task

• Check if all plugins are updated to NullSafety. If not then find a replacement.
• Move not null safe code to a new branch named no-null-safety. Keep sound null safety code in master.
• If the above task is completed Migrate current code to Sound Null Safety.