[IMPORTANT] fl_charts needs fix to work on Flutter 3.0.0

[kitsuniru]

Steps to Reproduce

Just update your flutter to 3.0.0

Versions

• which version of the Flutter are you using? - 3.0.0
• which version of the FlChart are you using? - 0.50.1

Actual results

/D:/flutter/.pub-cache/hosted/pub.dartlang.org/fl_chart-0.50.1/lib/src/chart/pie_chart/pie_chart.dart:42:20: Warning: Operand of null-aware operation '?.' has type 'WidgetsBinding' which excludes null.
 - 'WidgetsBinding' is from 'package:flutter/src/widgets/binding.dart' ('/D:/flutter/packages/flutter/lib/src/widgets/binding.dart').
    WidgetsBinding.instance?.addPostFrameCallback((_) {
                   ^