(Node)Error [ERR_HTTP_HEADERS_SENT]: Cannot set headers after they are sent to the client

I have been learninng NodeJS and mongoDB by youtube, but unfortunately i faced with this problem, and here is my code file! thank you in advance!

db.js

const { MongoClient } = require("mongodb");

let dbConnection;

module.exports = {
  connectToDb: (cb) => {
    MongoClient.connect("mongodb://localhost:27017/bookstore")
      .then((res) => {
        dbConnection = res.db();
        return cb();
      })
      .catch((error) => {
        console.log(error);
        return cb(error);
      });
  },
  getDb: () => dbConnection,
};

index.js

const express = require("express");
const { connectToDb, getDb } = require("./db");

// init app and middleware
const app = express();

//db connection
let db;

connectToDb((xato) => {
  if (!xato) {
    app.listen(3000, () => {
      console.log("The 3000 port is installed");
    });
    db = getDb();
    return db;
  }
});

//routes
app.get("/bookstore", (req, res) => {
  let mybook = [];
  // the collection name from mongoDB
  db.collection("bookstore")
    .find()
    .sort({ author: 1 })
    .forEach((book) => mybook.push(book))
    .then(() => {
      return res.sendStatus(200).json(mybook);
    })
    .catch(() => {
      return res.sendStatus(500).send("there were an error");
    });
  // res.json({ MyWords: "I am coming from json res" });
});

it must return data from local mongodb database. But it is facing with the problem. Please give me a solution!