Null safety support

[ronaldgosso]

/C:/flutter/.pub-cache/hosted/pub.dartlang.org/flare_flutter-3.0.2/lib/flare_render_box.dart:170:28: Warning: Operand of null-aware operation '?.' has type 'SchedulerBinding' which excludes null.

• 'SchedulerBinding' is from 'package:flutter/src/scheduler/binding.dart' ('/C:/flutter/packages/flutter/lib/src/scheduler/binding.dart').
  package:flutter/…/scheduler/binding.dart:1
  SchedulerBinding.instance?.scheduleFrameCallback(_beginFrame) ?? -1;