v1.0.0-nullsafety.1

Changes:

• Stable nullsafety release.
• BREAKING: Removed prevState from IndicatorController class.
  Because flutter only marks the widget that it is ready for rebuild, it is possible that the controller state will change more than once during a single frame what causes one or more steps to be skipped. To still use prevState and didChangeState method, you can use IndicatorStateHelper. Take a look at check_mark_indicator.dart or warp_indicator.dart for example usage.
• Added IndicatorStateHelper class.
• Added IndicatorController unit tests.
• Added warp indicator example.