Non-nullable instance field 'id' must be initialized, Non-nullable instance field '_database' must be initialized. in flutter

Question

could anyone help me with how to manage this problem? I am trying to code exactly likes the course's video, but I got this problem:

I'm getting (Non-nullable instance field 'id' must be initialized. Non-nullable instance field 'miles' must be initialized. Non-nullable instance field 'name' must be initialized. Non-nullable instance field '_database' must be initialized.) . errors in my project.

My car class:

import './dbhelper.dart';

class Car {
  int id;
  String name;
  int miles;

  Car(this.id, this.name, this.miles);

  **GET ERROR FROM Car.fromMap..**

  Car.fromMap(Map<String, dynamic> map) {

    ** Non-nullable instance field 'id' must be initialized.
    Non-nullable instance field 'miles' must be initialized.
    Non-nullable instance field 'name' must be initialized.** 

    id = map['id'];
    name = map['name'];
    miles = map['miles'];
  }

  Map<String, dynamic> toMap() {
    return {
      DatabaseHelper.columnId: id,
      DatabaseHelper.columnName: name,
      DatabaseHelper.columnMiles: miles,
    };
  }
}

My dbHelper.dart:

import './car.dart';
import 'package:path/path.dart';
import 'package:sqflite/sqflite.dart';

class DatabaseHelper {

  static final _databaseName = "cardb.db";
  static final _databaseVersion = 1;

  static final table = 'cars_table';

  static final columnId = 'id';
  static final columnName = 'name';
  static final columnMiles = 'miles';

  ** GET ERROR FROM DatabaseHelper._provateConstructor(); AND _database;
   Non-nullable instance field '_database' must be initialized.**

  DatabaseHelper._privateConstructor();
  static final DatabaseHelper instance = DatabaseHelper._privateConstructor();

  ** GET ERROR Non-nullable instance field '_database' must be initialized.** 

  static Database _database;
  Future<Database> get database async {
    if (_database != null) return _database;
    // lazily instantiate the db the first time it is accessed
    _database = await _initDatabase();
    return _database;
  }

  // this opens the database (and creates it if it doesn't exist)
  _initDatabase() async {
    String path = join(await getDatabasesPath(), _databaseName);
    return await openDatabase(path,
        version: _databaseVersion,
        onCreate: _onCreate);
  }

  // SQL code to create the database table
  Future _onCreate(Database db, int version) async {
    await db.execute('''
          CREATE TABLE $table (
            $columnId INTEGER PRIMARY KEY AUTOINCREMENT,
            $columnName TEXT NOT NULL,
            $columnMiles INTEGER NOT NULL
          )
          ''');
  }

  // Helper methods

  // Inserts a row in the database where each key in the Map is a column name
  // and the value is the column value. The return value is the id of the
  // inserted row.
  Future<int> insert(Car car) async {
    Database db = await instance.database;
    return await db.insert(table, {'name': car.name, 'miles': car.miles});
  }

  // All of the rows are returned as a list of maps, where each map is
  // a key-value list of columns.
  Future<List<Map<String, dynamic>>> queryAllRows() async {
    Database db = await instance.database;
    return await db.query(table);
  }

  // Queries rows based on the argument received
  Future<List<Map<String, dynamic>>> queryRows(name) async {
    Database db = await instance.database;
    return await db.query(table, where: "$columnName LIKE '%$name%'");
  }

  // All of the methods (insert, query, update, delete) can also be done using
  // raw SQL commands. This method uses a raw query to give the row count.
  Future<int> queryRowCount() async {
    Database db = await instance.database;
    return Sqflite.firstIntValue(await db.rawQuery('SELECT COUNT(*) FROM $table'));
  }

  // We are assuming here that the id column in the map is set. The other
  // column values will be used to update the row.
  Future<int> update(Car car) async {
    Database db = await instance.database;
    int id = car.toMap()['id'];
    return await db.update(table, car.toMap(), where: '$columnId = ?', whereArgs: [id]);
  }

  // Deletes the row specified by the id. The number of affected rows is
  // returned. This should be 1 as long as the row exists.
  Future<int> delete(int id) async {
    Database db = await instance.database;
    return await db.delete(table, where: '$columnId = ?', whereArgs: [id]);
  }
}

Wilson Toribio · Answer 1 · 2022-04-03T00:09:50.670

You have to put the null aware operator in the variables type for example:

class Car {
  int? id; // <---- add a ? to int so it will be int?
  String? name;  // <-- same here
  int? miles;  // <--- and here

  Car({this.id, this.name, this.miles});

  Car.fromMap(Map<String, dynamic> map) {

    id = map['id'];
    name = map['name'];
    miles = map['miles'];
  }

  Map<String, dynamic> toMap() {
    return {
      DatabaseHelper.columnId: id,
      DatabaseHelper.columnName: name,
      DatabaseHelper.columnMiles: miles,
    };
  }
}

Null safety as it is explain in the dart documentation:

When you opt into null safety, types in your code are non-nullable by default, meaning that variables can’t contain null unless you say they can. With null safety, your runtime null-dereference errors turn into edit-time analysis errors.

More information in: https://dart.dev/null-safety

Maybe the tutorial you are taking was previous the null aware implementation and it does not apply the null aware operators, you can do the same at the beggining of the type declartion where the error is happening in other classes, you just have to add ? to the variable type at the beginning of the variable.

Non-nullable instance field 'id' must be initialized, Non-nullable instance field '_database' must be initialized. in flutter

1 Answers1