Flutter复制官方例子“交互动画StatefulWidget”时出现以下报错？

Question

官方例子：

import 'package:flutter/material.dart';

//---------------------------- ParentWidget ----------------------------

class ParentWidget extends StatefulWidget {
  @override
  _ParentWidgetState createState() => new _ParentWidgetState();
}

class _ParentWidgetState extends State<ParentWidget> {
  bool _active = false;

  void _handleTapboxChanged(bool newValue) {
    setState(() {
      _active = newValue;
    });
  }

  @override
  Widget build(BuildContext context) {
    return new Container(
      child: new EntryPage(
        active: _active,
        onChanged: _handleTapboxChanged,
      ),
    );
  }
}

//----------------------------- TapboxC ------------------------------

class EntryPage extends StatefulWidget {
  EntryPage({Key key, this.active: false, @required this.onChanged})
      : super(key: key);

  final bool active;
  final ValueChanged<bool> onChanged;

  _TapboxCState createState() => new _TapboxCState();
}

class _TapboxCState extends State<EntryPage> {
  bool _highlight = false;

  void _handleTapDown(TapDownDetails details) {
    setState(() {
      _highlight = true;
    });
  }

  void _handleTapUp(TapUpDetails details) {
    setState(() {
      _highlight = false;
    });
  }

  void _handleTapCancel() {
    setState(() {
      _highlight = false;
    });
  }

  void _handleTap() {
    widget.onChanged(!widget.active);
  }

  Widget build(BuildContext context) {
    // This example adds a green border on tap down.
    // On tap up, the square changes to the opposite state.
    return new GestureDetector(
      onTapDown: _handleTapDown, // Handle the tap events in the order that
      onTapUp: _handleTapUp, // they occur: down, up, tap, cancel
      onTap: _handleTap,
      onTapCancel: _handleTapCancel,
      child: new Container(
        child: new Center(
          child: new Text(widget.active ? 'Active' : 'Inactive',
              style: new TextStyle(fontSize: 32.0, color: Colors.white)),
        ),
        width: 200.0,
        height: 200.0,
        decoration: new BoxDecoration(
          color:
          widget.active ? Colors.lightGreen[700] : Colors.grey[600],
          border: _highlight
              ? new Border.all(
            color: Colors.teal[700],
            width: 10.0,
          )
              : null,
        ),
      ),
    );
  }
}

报错代码

I/flutter ( 9953): ══╡ EXCEPTION CAUGHT BY GESTURE ╞═══════════════════════════════════════════════════════════════════
I/flutter ( 9953): The following NoSuchMethodError was thrown while handling a gesture:
I/flutter ( 9953): The method 'call' was called on null.
I/flutter ( 9953): Receiver: null
I/flutter ( 9953): Tried calling: call(true)
I/flutter ( 9953): When the exception was thrown, this was the stack:
I/flutter ( 9953): #0      Object.noSuchMethod (dart:core-patch/object_patch.dart:50:5)
I/flutter ( 9953): #1      _TapboxCState._handleTap (package:flutter_app/entry.dart:64:12)
I/flutter ( 9953): #2      GestureRecognizer.invokeCallback (package:flutter/src/gestures/recognizer.dart:166:24)
I/flutter ( 9953): #3      TapGestureRecognizer._checkUp (package:flutter/src/gestures/tap.dart:240:9)
I/flutter ( 9953): #4      TapGestureRecognizer.handlePrimaryPointer (package:flutter/src/gestures/tap.dart:177:9)
I/flutter ( 9953): #5      PrimaryPointerGestureRecognizer.handleEvent (package:flutter/src/gestures/recognizer.dart:436:9)
I/flutter ( 9953): #6      PointerRouter._dispatch (package:flutter/src/gestures/pointer_router.dart:73:12)
I/flutter ( 9953): #7      PointerRouter.route (package:flutter/src/gestures/pointer_router.dart:101:11)
I/flutter ( 9953): #8      _WidgetsFlutterBinding&BindingBase&GestureBinding.handleEvent (package:flutter/src/gestures/binding.dart:221:19)
I/flutter ( 9953): #9      _WidgetsFlutterBinding&BindingBase&GestureBinding.dispatchEvent (package:flutter/src/gestures/binding.dart:199:22)
I/flutter ( 9953): #10     _WidgetsFlutterBinding&BindingBase&GestureBinding._handlePointerEvent (package:flutter/src/gestures/binding.dart:156:7)
I/flutter ( 9953): #11     _WidgetsFlutterBinding&BindingBase&GestureBinding._flushPointerEventQueue (package:flutter/src/gestures/binding.dart:102:7)
I/flutter ( 9953): #12     _WidgetsFlutterBinding&BindingBase&GestureBinding._handlePointerDataPacket (package:flutter/src/gestures/binding.dart:86:7)
I/flutter ( 9953): #16     _invoke1 (dart:ui/hooks.dart:233:10)
I/flutter ( 9953): #17     _dispatchPointerDataPacket (dart:ui/hooks.dart:154:5)
I/flutter ( 9953): (elided 3 frames from package dart:async)
I/flutter ( 9953): Handler: onTap
I/flutter ( 9953): Recognizer:
I/flutter ( 9953):   TapGestureRecognizer#cb503(debugOwner: GestureDetector, state: possible, won arena, finalPosition:
I/flutter ( 9953):   Offset(259.0, 456.0), sent tap down)
I/flutter ( 9953): ════════════════════════════════════════════════════════════════════════════════════════════════════