这是我得到的错误：
openUri()的uri参数必须是一个字符串，得到“undefined”。确保mongoose.connect()或mongoose.createConnection()的第一个参数是字符串。未连接
服务器文件夹中我的.env文件：

MONGO_URL='mongodb+srv://dummyuser:Abhinav@2170@cluster0.e7kxje8.mongodb.net/?retryWrites=true&w=majority'
PORT=3001

服务器文件夹中的index.js文件：

import express from "express";
import bodyParser from "body-parser";
import mongoose from "mongoose";
import cors from "cors";
import dotenv from "dotenv";
import multer from "multer";
import helmet from "helmet";
import morgan from "morgan";
import path from "path";
import { fileURLToPath } from "url";

/* CONFIGURATIONS */
const __filename = fileURLToPath(import.meta.url);
const __dirname = path.dirname(__filename);
dotenv.config();
const app = express();
app.use(express.json());
app.use(helmet());
app.use(helmet.crossOriginResourcePolicy({ policy: "cross-origin" }));
app.use(morgan("common"));
app.use(bodyParser.json({ limit: "30mb", extended: true }));
app.use(bodyParser.urlencoded({ limit: "30mb", extended: true }));
app.use(cors());
app.use("/assets", express.static(path.join(__dirname, "public/assets")));

/* FILE STORAGE */
const storage = multer.diskStorage({
    destination: function (req, file, cb) {
      cb(null, "public/assets");
    },
    filename: function (req, file, cb) {
      cb(null, file.originalname);
    },
  });
  const upload = multer({ storage });

/* MONGOOSE SETUP */
const PORT = process.env.PORT || 6001;
mongoose
  .connect(process.env.MONGO_URL, {
    useNewUrlParser: true,
    useUnifiedTopology: true,
  })
  .then(() => {
    app.listen(PORT, () => console.log(`Server Port: ${PORT}`));
  })
  .catch((error) => console.log(`${error} did not connect`));