在TextField中打印'newTaskTitle‘的值时，它工作得很好。但是当我尝试从FlatButton打印它时，它显示为'null‘。

import 'package:flutter/material.dart';
import 'package:provider/provider.dart';

void main() {
  runApp(MyApp());
}

class TaskData extends ChangeNotifier {
  String newTaskTitle;

  void updateTaskTitle(String title) {
    newTaskTitle = title;
    notifyListeners();
  }
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return ChangeNotifierProvider(
      create: (context) => TaskData(),
      child: MaterialApp(
        home: MyHomePage(),
      ),
    );
  }
}

class MyHomePage extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('My App'),
      ),
      body: TextField(
        onChanged: (newValue) {
          Provider.of<TaskData>(context, listen: false).updateTaskTitle(newValue);
        },
      ),
      floatingActionButton: FlatButton(
        onPressed: () {
          String taskTitle = Provider.of<TaskData>(context, listen: false).newTaskTitle;
          print(taskTitle);
        },
        child: Text('Print Task Title'),
      ),
    );
  }
}