I am new in MongoDB. When I run the code below, I get an errror: Topology is closed. I want to create a document in collection. Why does this happen and how can I fix it? I would appretiate any help.

const { MongoClient } = require('mongodb');
const uri = "mongodb+srv://(login info)@cluster0.56skb.mongodb.net/myFirstDatabase?retryWrites=true&w=majority";
const client = new MongoClient(uri, { useNewUrlParser: true, useUnifiedTopology: true });
client.connect(err => {
 const db = client.db("test");
 async function mongo () {
   await db.collection('a').insertMany([
  { item: "journal",
    qty: 25,
    size: { h: 14, w: 21, uom: "cm" },
    status: "A"},
  { item: "notebook",
    qty: 50,
    size: { h: 8.5, w: 11, uom: "in" },
    status: "A"},
  { item: "paper",
    qty: 100,
    size: { h: 8.5, w: 11, uom: "in" },
    status: "D"},
  { item: "planner",
    qty: 75, size: { h: 22.85, w: 30, uom: "cm" },
    status: "D"},
  { item: "postcard",
    qty: 45,
    size: { h: 10, w: 15.25, uom: "cm" },
    status: "A"}
])
.then(function(result) {
  // process result
})
 }
  mongo()
const collection = client.db("test").collection("devices");
  // perform actions on the collection object
  client.close();
});