Add Null Safety To Dart Repo. Migrate to null safe dart sdk

[akashgk]

We need to make the repo null safe, so we are up to date with the latest coding standards in dart.

We also need to migrate the files to follow null safety.

List of tasks.

• [x] Fix the dart analysis errors.
• [ ] update dart sdk
• [ ] Update Dart SDK to 2.18 or higher.
• [ ] Run Dart Migrate.
• [ ] fix code in files with older sdk
• [ ] Test Changes.