Flutter 3.0.0 migration backward compatibility fix (#31)

Author: RBT22

example/lib/indicators/warp_indicator.dart

@@ -5,6 +5,14 @@ import 'package:custom_refresh_indicator/custom_refresh_indicator.dart';
 import 'package:flutter/rendering.dart';
 import 'package:flutter/scheduler.dart';
 
+/// This allows a value of type T or T?
+/// to be treated as a value of type T?.
+///
+/// We use this so that APIs that have become
+/// non-nullable can still be used with `!` and `?`
+/// to support older versions of the API as well.
+T? _ambiguate<T>(T? value) => value;
+
 enum WarpAnimationState {
   stopped,
   playing,
@@ -163,7 +171,7 @@ class _WarpIndicatorState extends State<WarpIndicator>
                   child: Builder(builder: (context) {
                     if (shakeController.value == 1.0 &&
                         _state == WarpAnimationState.playing) {
-                      SchedulerBinding.instance
+                      _ambiguate(SchedulerBinding.instance)!
                           .addPostFrameCallback((_) => _resetShakeAnimation());
                     }
                     return Transform.rotate(
@@ -303,6 +311,7 @@ class Sky extends CustomPainter {
 
   @override
   bool shouldRepaint(Sky oldDelegate) => true;
+
   @override
   bool shouldRebuildSemantics(Sky oldDelegate) => false;
 }

lib/src/custom_refresh_indicator.dart

@@ -1,5 +1,13 @@
 part of custom_refresh_indicator;
 
+/// This allows a value of type T or T?
+/// to be treated as a value of type T?.
+///
+/// We use this so that APIs that have become
+/// non-nullable can still be used with `!` and `?`
+/// to support older versions of the API as well.
+T? _ambiguate<T>(T? value) => value;
+
 typedef IndicatorBuilder = Widget Function(
   BuildContext context,
   Widget child,
@@ -206,7 +214,7 @@ class CustomRefreshIndicatorState extends State<CustomRefreshIndicator>
 
     if (_canStart) setIndicatorState(IndicatorState.dragging);
 
-    WidgetsBinding.instance.addPostFrameCallback((_) {
+    _ambiguate(WidgetsBinding.instance)!.addPostFrameCallback((_) {
       controller.setAxisDirection(notification.metrics.axisDirection);
     });