我在Youtube上学习了this教程，介绍了如何在Flutter应用程序中创建数据库，我严格按照教程中的所有说明操作，但每当我尝试单击保存按钮将新注解保存到数据库时，都会出现此异常，这表明NOT NULL constraint failed .

在这里是我的文件的样子：note_model.dart：

class Note {
  final int? id;
  final String title;
  final String content;

  const Note({required this.title, required this.content, this.id});

  factory Note.fromJson(Map<String, dynamic> json) => Note(
        id: json['id'],
        title: json['title'],
        content: json['content'],
      );

  Map<String, dynamic> toJson() => {
        'id': id,
        'title': title,
        'content': content,
      };

  @override
  String toString() {
    return "id: $id \n title: $title \n  content: $content \n";
  }
}

database_help：

import 'package:dummy_database/models/note_model.dart';
import 'package:sqflite/sqflite.dart';
import 'package:path/path.dart';

class DatabaseHelper {
  static const int _version = 1;
  static const String _dbName = "Notes.db";

  static Future<Database> _getDB() async {
    return openDatabase(join(await getDatabasesPath(), _dbName),
        onCreate: (db, version) async => await db.execute(
            "CREATE TABLE Note(id INTEGER PRIMARY KEY AUTOINCREMENT, title TEXT NOT NULL, content TEXT NOT NULL);"),
        version: _version);
  }

  static Future<int> addNote(Note note) async {
    final db = await _getDB();
    return await db.insert("Note", note.toJson(),
        conflictAlgorithm: ConflictAlgorithm.replace);
  }

  static Future<int> updateNote(Note note) async {
    final db = await _getDB();
    return await db.update("Note", note.toJson(),
        where: 'id = ?',
        whereArgs: [note.id],
        conflictAlgorithm: ConflictAlgorithm.replace);
  }

  static Future<int> deleteNote(Note note) async {
    final db = await _getDB();
    return await db.delete(
      "Note",
      where: 'id = ?',
      whereArgs: [note.id],
    );
  }

  static Future<List<Note>?> getAllNotes() async {
    final db = await _getDB();

    final List<Map<String, dynamic>> maps = await db.query("Note");

    if (maps.isEmpty) {
      return null;
    }

    return List.generate(maps.length, (index) => Note.fromJson(maps[index]));
  }
}

以下是导致异常的文件：

import 'dart:developer';

import 'package:dummy_database/models/note_model.dart';
import 'package:dummy_database/services/database_helper.dart';
import 'package:flutter/material.dart';

class NoteScreen extends StatelessWidget {
  final Note? note;
  NoteScreen({Key? key, this.note}) : super(key: key);

  final titleController = TextEditingController();
  final contentController = TextEditingController();

  @override
  Widget build(BuildContext context) {
    if (note != null) {
      titleController.text = note!.title;
      contentController.text = note!.content;
    }

    return Scaffold(
      appBar: AppBar(
        title: Text(note == null ? 'Add a note' : 'Edit note'),
        centerTitle: true,
      ),
      body: Padding(
        padding: const EdgeInsets.symmetric(vertical: 10, horizontal: 30),
        child: Column(
          children: [
            const Padding(
              padding: EdgeInsets.only(bottom: 40),
              child: Center(
                child: Text(
                  'What are you thinking about?',
                  style: TextStyle(fontSize: 20, fontWeight: FontWeight.bold),
                ),
              ),
            ),
            Padding(
              padding: const EdgeInsets.only(bottom: 40.0),
              child: TextFormField(
                controller: titleController,
                maxLines: 1,
                decoration: const InputDecoration(
                    hintText: 'Title',
                    labelText: 'Note title',
                    border: OutlineInputBorder(
                        borderSide: BorderSide(
                          color: Colors.white,
                          width: 0.75,
                        ),
                        borderRadius: BorderRadius.all(
                          Radius.circular(10.0),
                        ))),
              ),
            ),
            TextFormField(
              controller: contentController,
              decoration: const InputDecoration(
                  hintText: 'Type here the note',
                  labelText: 'Note content',
                  border: OutlineInputBorder(
                      borderSide: BorderSide(
                        color: Colors.white,
                        width: 0.75,
                      ),
                      borderRadius: BorderRadius.all(
                        Radius.circular(10.0),
                      ))),
              keyboardType: TextInputType.multiline,
              onChanged: (str) {},
              maxLines: 5,
            ),
            const Spacer(),
            Padding(
              padding: const EdgeInsets.only(bottom: 20.0),
              child: SizedBox(
                height: 45,
                width: MediaQuery.of(context).size.width,
                child: ElevatedButton(
                    onPressed: () async {
                      final title = titleController.value.text;
                      final content = contentController.value.text;

                      log("title value is $title, \n content value is $content");
                      if (title.isEmpty || content.isEmpty) {
                        return;
                      }

                      final Note model =
                          Note(title: title, content: content, id: note?.id);
                          log(model.toString());
                      if (note == null) {
                        await DatabaseHelper.addNote(model);
                      } else {
                        await DatabaseHelper.updateNote(model);
                      }

                      Navigator.pop(context);
                    },
                    style: ButtonStyle(
                        shape: MaterialStateProperty.all(
                            const RoundedRectangleBorder(
                                side: BorderSide(
                                  color: Colors.white,
                                  width: 0.75,
                                ),
                                borderRadius: BorderRadius.all(
                                  Radius.circular(10.0),
                                )))),
                    child: Text(
                      note == null ? 'Save' : 'Edit',
                      style: const TextStyle(fontSize: 20),
                    )),
              ),
            )
          ],
        ),
      ),
    );
  }
}