无法解决“空值检查运算符用于空值”的问题

Question

我正在尝试在 flutter 中创建一个待办事项列表应用程序。我面临一个小部件的问题，该小部件可以帮助我编辑已创建的待办事项小部件的标题或描述。

这是我的待办事项列表主页的图片

我的编辑按钮的图片

我在按后遇到问题此小部件中的保存按钮

这是我收到的错误

════════ Exception caught by gesture ═══════════════════════════════════════════
The following _CastError was thrown while handling a gesture:
Null check operator used on a null value
When the exception was thrown, this was the stack
#0      _EditTodoPageState.saveTodo
package:todolist/Edit_Todo_Page.dart:47
#1      _InkResponseState._handleTap
package:flutter/…/material/ink_well.dart:989
#2      GestureRecognizer.invokeCallback
package:flutter/…/gestures/recognizer.dart:198
#3      TapGestureRecognizer.handleTapUp
package:flutter/…/gestures/tap.dart:608
#4      BaseTapGestureRecognizer._checkUp
package:flutter/…/gestures/tap.dart:296
#5      BaseTapGestureRecognizer.handlePrimaryPointer
package:flutter/…/gestures/tap.dart:230
#6      PrimaryPointerGestureRecognizer.handleEvent
package:flutter/…/gestures/recognizer.dart:563
#7      PointerRouter._dispatch
package:flutter/…/gestures/pointer_router.dart:94
#8      PointerRouter._dispatchEventToRoutes.<anonymous closure>
package:flutter/…/gestures/pointer_router.dart:139
    (elided 3 frames from dart:async)
Handler: "onTap"
Recognizer: TapGestureRecognizer#25bd2
    debugOwner: GestureDetector
    state: possible
    won arena
    finalPosition: Offset(189.1, 309.1)
    finalLocalPosition: Offset(67.7, 11.1)
    button: 1
    sent tap down
════════════════════════════════════════════════════════════════════════════════

这是我的编辑待办事项小部件

import 'package:flutter/material.dart';
import 'package:provider/provider.dart';
import 'package:todolist/todo.dart';
import './todos.dart';
import './todo_form_widget.dart';

class EditTodoPage extends StatefulWidget {
  final Todo todo;
  const EditTodoPage({Key? key, required this.todo}) : super(key: key);

  @override
  State<EditTodoPage> createState() => _EditTodoPageState();
}

class _EditTodoPageState extends State<EditTodoPage> {
  final _formKey = GlobalKey<FormState>();
  late String title;
  late String description;
  @override
  void initState() {
    super.initState();
    title = widget.todo.title;
    description = widget.todo.description;
  }

  Widget build(BuildContext context) {
    return Scaffold(
        appBar: AppBar(
          title: Text("Edit Todo"),
        ),
        body: Padding(
          padding: EdgeInsets.all(16),
          child: Form(
            child: TodoFormWidget(
              title: title,
              description: description,
              onChangedTitle: (title) => setState(() => this.title = title),
              onChangedDescription: (description) =>
                  setState(() => this.description = description),
              onSavedTodo: saveTodo,
            ),
          ),
        ));
  }

  saveTodo() {
    final isValid = _formKey.currentState!.validate();
    if (!isValid) {
      return;
    } else {
      final provider = Provider.of<TodosProvider>(context, listen: false);
      provider.updateTodo(widget.todo, title, description);
      Navigator.of(context).pop();
    }
  }
}

我在“saveTodo()”方法中收到错误。我无法解决这个错误。

这是我的 Provider 小部件

import 'package:flutter/cupertino.dart';
import 'package:todolist/todo.dart';
import 'package:provider/provider.dart';

class TodosProvider extends ChangeNotifier {
  List<Todo> _todos = [
    Todo(
      createdTime: DateTime.now(),
      title: "Wake up at 6:00 AM in the morning
              

Answer 1

我认为，您没有在 Form 小部件中使用 _formKey，这就是 _formKey currentState 变为 null 的原因。尝试在 Form 小部件中使用 _formKey 并检查它不等于 null。

import 'package:flutter/material.dart';
import 'package:provider/provider.dart';
import 'package:todolist/todo.dart';
import './todos.dart';
import './todo_form_widget.dart';

class EditTodoPage extends StatefulWidget {
  final Todo todo;
  const EditTodoPage({Key? key, required this.todo}) : super(key: key);

  @override
  State<EditTodoPage> createState() => _EditTodoPageState();
}

class _EditTodoPageState extends State<EditTodoPage> {
  final _formKey = GlobalKey<FormState>();
  late String title;
  late String description;

  @override
  void initState() {
    super.initState();
    title = widget.todo.title;
    description = widget.todo.description;
  }

  Widget build(BuildContext context) {
    return Scaffold(
        appBar: AppBar(
          title: Text("Edit Todo"),
        ),
        body: Padding(
          padding: EdgeInsets.all(16),
          child: Form(
            key: _formKey,
            child: TodoFormWidget(
              title: title,
              description: description,
              onChangedTitle: (title) => setState(() => this.title = title),
              onChangedDescription: (description) =>
                  setState(() => this.description = description),
              onSavedTodo: saveTodo,
            ),
          ),
        ),
     );
  }

  saveTodo() {
    final isValid = _formKey.currentState != null && _formKey.currentState!.validate();
    if (!isValid) {
      return;
    } else {
      final provider = Provider.of<TodosProvider>(context, listen: false);
      provider.updateTodo(widget.todo, title, description);
      Navigator.of(context).pop();
    }
  }
}