当我运行以下代码时，我收到错误消息"MongoError: server instance pool was destroyed".知道为什么或者如何解决这个问题吗？
var csv = require('./importer.js');
var MongoClient = require('mongodb').MongoClient;
var assert = require('assert');
var ObjectId = require('mongodb').ObjectID;
var url = 'mongodb://.....';
MongoClient.connect(url, function(err, db) {
assert.equal(null, err);
console.log("Connected correctly to server.");
csv.foreach('data/airports.csv', function(airport){
db.collection('airports').insertOne(airport, function(err, result) {
if(err) {
console.log(err)
} else {
console.log("Inserted: " + airport.ident);
}
});
});
db.close();
});