null-safety

[barbalex]

Please migrate to using null safety: https://dart.dev/null-safety/migration-guide

Dependencies:

• There is a null safe version of flutter_map published now (0.13.0) but there is a major bug that causes the map to crash as soon as you move/rotate: fix: remove final from none-final fields fleaflet/flutter_map#923.
• dio: ^4.0.0 is null safe (there seem to be some breaking changes though: https://pub.dev/packages/dio/changelog).
• permission_handler: ^8.0.1 is current, null safe since 6.0.0.
• flutter_range_slider: ^1.5.0 is not null safe yet. There is an open pull request: Migrate to null safety boeledi/RangeSlider#35.
• path_provider: ^2.0.2 is null safe.
• shared_preferences: ^2.0.6 is null safe (since 2.0.0).
• percent_indicator: ^3.0.1 is null safe.