Change non-nullsafe tripledes package to a version that is nullsafe

[harrybawsac]

This pull request changes the Triple DES dependency from this one (which is not null safe) to this one.

~Also updated the code example to make use of flutter_platform_widgets: ^2.0.0. And thus the PlatformTextButton is replacing the PlatformButton in main.dart of the example.~

It also changes some of the code formatting to make it a bit more uniform and in line with Dart/Flutter code formatting principles.

I tested the example project on a Nokia 7.1, which works.

[smlu]

Thank you for the contribution!

I see that commit fd6fbcea in PR reverts the update of example app to the new platform widgets. What's the reason for this?

[harrybawsac]

It was just a small update of a dependency of the example app. But the formatting went nuts on Ctrl + S, so it turned into this insane, unreviewable PR for such a minor addition. Thought I'd just stick to the small null safety stuff instead.

[smlu]

Ok, could you please squash commits to have single commit or just force push commit 1e28a44e.

[harrybawsac]

Done :heavy_check_mark:

[smlu]

Thank you! Merging now this PR.