Mongodb UseUnifiedTopology: True causes connection errors in my code - node.js

Question

I have a small app written in Node.js that downloads some data from an API, and then saves it into a MongoDB. When I have useunifiedtopology: false my code runs fine, connects to the mongodb fine and updates everything fine.
When I set useunifiedtopolgy: true I get the following errors
MongoError: Cannot use a session that has ended
Here is what I am trying to accomplish...I can't for the life of me figure out where it's going wrong. If I remove the db.close() from dbupdater.js, it works, but the app just hangs.
Here is how I am going about things...
It should also be noted, that if I remove the multiple locations and extraction calls from below, then it does work fine. The errors noted above when using useunifiedtopology: false only seem to happen when I am doing multiple calls to the dbupdater module. So, to be clear, if I just hit the API once, and do one call to dbupdater, it works fine. If I call the API and the dbupdater function multiple times, it fails with the error:
MongoError: Cannot use a session that has ended.
app.js Contents:
const Downloader = require('./modules/Downloader')
const dbupdater = require('./modules/dbUpdater')
var Locations = ['Location1','Location2']
var ExtractionTypes = ['Extraction1','Extraction2','Extraction3' ]
var interval = 60000; //API Request Delay in MS
var promise = Promise.resolve();
function main() {
Locations.forEach(location => {
promise = promise.then(function(){
ExtractionTypes.forEach(extraction =>{
promise = promise.then(function(){
talonDownloader(location,extraction,function(data){
if (data == null) {
console.log(`No ${extraction} Data Recieved for period requested.`)
}
else {
dbupdater(extraction,data)
}
})
return new Promise(function (resolve) {
setTimeout(resolve, interval)
})
})
})
return new Promise(function (resolve) {
setTimeout(resolve, interval)
})
})
})
}
main();
Then here is the code for my dbupdater...
dbudater.js
module.exports = function (extraction,data) {
const MongoClient = require('mongodb').MongoClient;
const date = require('date-and-time');
const crypto = require('crypto')
const dburl = "mongodb://localhost:27017/";
const dbName = 'palDB'
//Initialize Date Information
const now = new Date()
const todayDate = date.format(now, 'DD-MMM-YYYY')
if (extraction == 'operation') {
var collectionName = 'operations'
var indexValue = 'ACTIVITY_ID'
updateDBGeneral();
}
else if (extraction == 'aircraft') {
var collectionName = 'aircraft'
var indexValue = 'AIRCRAFT_ID'
updateDBGeneral();
updateDBHistorical();
}
else if (extraction == 'cancelanalysis') {
var collectionName = 'cancelAnalysis'
var indexValue = 'SCH_ACT_ID'
updateDBGeneral();
}
else if (extraction == 'dailyflylog') {
var collectionName = 'dfl'
var indexValue = 'SCH_ACT_ID'
updateDBGeneral();
}
else if (extraction == 'instructor') {
var collectionName = 'instructor'
var indexValue = 'ETA_ID'
updateDBGeneral();
}
else if (extraction == 'resschedproductivity') {
var collectionName = 'resSchedProductivity'
var indexValue = 'opid'
updateDBGeneral();
}
else if (extraction == 'student') {
var collectionName = 'student'
var indexValue = 'ETA_ID'
updateDBGeneral();
}
else if (extraction == 'trainingrecord') {
var collectionName = 'trgrec'
var indexValue = 'STUDENT_REGISTRATION_ID'
updateDBGeneral();
}
else if (extraction == 'studentprogression') {
var collectionName = 'studentProgression'
var indexValue = 'studentProgIndex'
updateDBGeneral();
}
//Initialize DB Updater functions
function updateDBGeneral(){
MongoClient.connect(dburl,{useNewUrlParser: true, useUnifiedTopology: true})
.then((db) => {
var dbo = db.db(dbName);
console.log(`Connection Established to ${dbo.databaseName} database.`)
data.forEach(element => {
if (indexValue == 'opid') {
element[indexValue] = crypto.createHash('md5').update(element.CAL_DATE + element.RES_TYPE_ID).digest('hex'); //Add Value to filter on
var filter = {[indexValue]: element[indexValue]};
} else if (indexValue =='studentProgIndex'){
element[indexValue] = crypto.createHash('md5').update(element.PERS_REGISTER_ID + element.UNIT + element.ACT_START).digest('hex'); //Add value to filter on
var filter = {[indexValue]: element[indexValue]}
} else var filter = {[indexValue]: element[indexValue]}
dbo.collection(collectionName)
.updateOne(filter, {$set:element},{upsert:true})
.then( result => db.close())
.catch( err => console.log(err));
})
})
.catch( err => console.log(err));
}
function updateDBHistorical(){
}
}