我如何定义我的函数nullable？我在Flutter上得到以下错误（InsertData是repository.dart中的函数）：
lib/services/categoriesservices.dart：13：30：错误：无法访问“存储库？”上的属性“InsertData”，因为它可能为null。 'Repository'来自'package：sqflite 2/repositories/repositories.dart'（'lib/repositories/repositories. dart'）。请尝试使用？. instead. return await _repository.InsertData.call（

repository.dart如下所示：

import 'package:sqflite/sqflite.dart';
import 'package:sqflite2/repositories/databaseconnection.dart';

class Repository {
  DataBaseConnection? _dataBaseConnection;

  Repository() {
    //initialize database connection
    _dataBaseConnection = DataBaseConnection();
  }

  static Database? _database;
  Future<Database?> get database async {
    if (_database != null) {
      return _database;
    }
    _database = await _dataBaseConnection.setDatabase();
    return database;
  }

  //create function inserting data to database
  InsertData(table, data) async {
    var connection = await database;
    return await connection.insert(table, data);
  }
}

函数初始化如下所示：

import 'package:sqflite2/models/category.dart';
import 'package:sqflite2/repositories/repository.dart';

class CategoryService {
  Repository? _repository;

  CategoryService() {
    _repository = Repository();
  }

  saveCategory(Categori category) async {
    return await _repository.InsertData("categories", category.categoryMap());
  }
}

我错过了什么？我以为我已经用（？）初始化了存储库