BloC - 状态管理 - 红色错误框

BloC - 状态管理 - 红色错误框。

// !!!ОШИБКА ЗДЕСЬ!!!
final ColorBloc _bloc = BlocProvider.of<ColorBloc>(context);

告诉我如何更改此行以使没有错误？接下来是完整代码和有错误的屏幕截图。

主要.dart

import 'dart:async';

import 'package:flutter/material.dart';

import 'package:flutter_bloc/flutter_bloc.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
 @override
 Widget build(BuildContext context) {
  return MaterialApp(
   debugShowCheckedModeBanner: false,
   title: 'Name App',
   theme: ThemeData(
    primarySwatch: Colors.blue,
   ),
   home: MyHomePage(),
  );
 }
}

class MyHomePage extends StatelessWidget {

 @override
 Widget build(BuildContext context) {

   // !!!ОШИБКА ЗДЕСЬ!!!
   final ColorBloc _bloc = BlocProvider.of<ColorBloc>(context);

  return Scaffold(
   appBar: AppBar(
    title: Text('Name Page'),
   ),
   body: Stack(
    children: [
     Align(
      alignment: Alignment(0.0, -0.75),
      child: BlocBuilder<ColorBloc, Color>(
       //cubit: ColorBloc(Colors.red),
       builder: (context, state) {
        return AnimatedContainer(
         width: 150.0,
         height: 150.0,
         color: state,
         duration: Duration(milliseconds: 500),
        );
       },
      ),
     ),
     Align(
      alignment: Alignment(-1.0, 0.95),
      child: Row(
       mainAxisSize: MainAxisSize.max,
       mainAxisAlignment: MainAxisAlignment.start,
       children: [
        SizedBox(
         width: 15.0,
        ),
        Expanded(
         flex: 1,
         child: Container(
          height: 50.0,
          child: FlatButton(
           onPressed: () { _bloc.add(ColorEvent.event_red); },
           splashColor: Colors.blue.withOpacity(0.5),
           shape: RoundedRectangleBorder(
            borderRadius: BorderRadius.circular(10)
           ),
           child: Text(
            'OK',
            style: TextStyle(
             color: Colors.white,
             fontSize: 20.0,
            ),
           ),
           color: Colors.red,
          ),
         ),
        ),
        SizedBox(
         width: 15.0,
        ),
        Expanded(
         flex: 1,
         child: Container(
          height: 50.0,
          child: FlatButton(
           onPressed: () { _bloc.add(ColorEvent.event_green); },
           splashColor: Colors.blue.withOpacity(0.5),
           shape: RoundedRectangleBorder(
            borderRadius: BorderRadius.circular(10)
           ),
           child: Text(
            'OK',
            style: TextStyle(
             color: Colors.white,
             fontSize: 20.0,
            ),
           ),
           color: Colors.green,
          ),
         ),
        ),
        SizedBox(
         width: 15.0,
        ),
       ],
      ),
     ),
    ],
   ),
  );
 }

}

enum ColorEvent {event_red, event_green}

class ColorBloc extends Bloc<ColorEvent, Color> {
 ColorBloc(Color initialState) : super(initialState);

 Color _color = Colors.red;

 Color get initialState => Colors.red;

 @override
 Stream<Color> mapEventToState(ColorEvent event) async* {
  /*switch(event) {
    case ColorEvent.event_red:
     _color = Colors.red;
     yield _color;
     break;
    case ColorEvent.event_green:
     _color = Colors.green;
     yield _color;
     break;
   }*/
  _color = (event == ColorEvent.event_red) ? Colors.red : Colors.green;
  yield _color;
 }

}