#201 - Null-Safety support

[brenodt]

This PR aims to bring null-safety support to the package - as per #201 request.

The approach taken was to change as little as possible in terms of logic:

• Places where @required tag was used were replaced with required keyword and became non-nullable.
• Parameters that are checked for nullability somewhere down the line were set as nullable.
• If none of these two scenarios applied, used a default value.

On slider.dart and range_slider.dart, the onPanUpdate method declarations have had to be updated, as context.findRenderObject() no longer yields a RenderBox
(lib/src/widget/slider.dart:143, lib/src/widget/range_slider.dart:168). Instead, the tap position is now using DragUpdateDetails.localPosition.

Secondly, I couldn't ascertain how to regenerate the lib/generated/i18n.dart file, but I don't see it being used anywhere. Maybe it should be dropped?

@florent37 I hope this helps bringing the package to Null-Safety, let me know if you think I should change anything. Thanks in advance!

[7flash]

Please merge @florent37

[7flash]

I am getting this error when trying to run app depending on your branch brenodt:158-null-safety-support

[7flash]

This works - please fix @brenodt

  static double? embossDepth(BuildContext context) {
    return currentTheme(context).depth != null
        ? -(currentTheme(context).depth!.abs())
        : null;
  }

[brenodt]

@7flash Thanks for the review, fix made.

[florent37]

wow thanks a lot guys, I merge it !

[florent37]

I will publish a new verison <3