Facebook Messenger Bot : Socket hang up exception while sending message back to user using nodejs?

I have taken a example fb mot from git set up the node js app and subscribed to webhooks.

Below is the code :

'use strict';

const express = require('express')
const bodyParser = require('body-parser')
const request = require('request')
const app = express()
const fs = require('fs')
const https = require('https')

app.set('port', (process.env.PORT || 5000))

// parse application/x-www-form-urlencoded
app.use(bodyParser.urlencoded({extended: false}))

// parse application/json
app.use(bodyParser.json())

// index
app.get('/', function (req, res) {
    res.send('hello world i am a secret bot')
})

// for facebook verification
app.get('/webhook/', function (req, res) {
    if (req.query['hub.verify_token'] === 'my_voice_is_my_password_verify_me') {
        res.send(req.query['hub.challenge'])
console.log("verified");
    } else {
        res.send('Error, wrong token')
    }
})

// to post data
app.post('/webhook/', function (req, res) {
    let messaging_events = req.body.entry[0].messaging
    for (let i = 0; i < messaging_events.length; i++) {
        let event = req.body.entry[0].messaging[i]
        let sender = event.sender.id
        if (event.message && event.message.text) {
            let text = event.message.text
            if (text === 'Generic'){ 
                console.log("welcome to chatbot")
                //sendGenericMessage(sender)
                continue
            }
            sendTextMessage(sender, "Text received, echo: " + text.substring(0, 200))
        }
        if (event.postback) {
            let text = JSON.stringify(event.postback)
            sendTextMessage(sender, "Postback received: "+text.substring(0, 200), token)
            continue
        }
    }
    res.send(200)
})


// recommended to inject access tokens as environmental variables, e.g.
// const token = process.env.FB_PAGE_ACCESS_TOKEN
const token = "<TOKEN>"

function sendTextMessage(sender, text) {
    let messageData = { text:text }

    request({
        url: 'https://graph.facebook.com/v2.6/me/messages',
        qs: {access_token:token},
        method: 'POST',
        json: {
            recipient: {id:sender},
            message: messageData,
        }
    }, function(error, response, body) {
        if (error) {
            console.log('Error sending messages: ', error)
        } else if (response.body.error) {
            console.log('Error: ', response.body.error)
        }
    })
}

function sendGenericMessage(sender) {
    let messageData = {
        "attachment": {
            "type": "template",
            "payload": {
                "template_type": "generic",
                "elements": [{
                    "title": "First card",
                    "subtitle": "Element #1 of an hscroll",
                    "image_url": "http://messengerdemo.parseapp.com/img/rift.png",
                    "buttons": [{
                        "type": "web_url",
                        "url": "https://www.messenger.com",
                        "title": "web url"
                    }, {
                        "type": "postback",
                        "title": "Postback",
                        "payload": "Payload for first element in a generic bubble",
                    }],
                }, {
                    "title": "Second card",
                    "subtitle": "Element #2 of an hscroll",
                    "image_url": "http://messengerdemo.parseapp.com/img/gearvr.png",
                    "buttons": [{
                        "type": "postback",
                        "title": "Postback",
                        "payload": "Payload for second element in a generic bubble",
                    }],
                }]
            }
        }
    }
    request({
        url: 'https://graph.facebook.com/v2.6/me/messages',
        qs: {access_token:token},
        method: 'POST',
        json: {
            recipient: {id:sender},
            message: messageData,
        }
    }, function(error, response, body) {
        if (error) {
            console.log('Error sending messages: ', error)
        } else if (response.body.error) {
            console.log('Error: ', response.body.error)
        }
    })
}

// Start server
// Webhooks must be available via SSL with a certificate signed by a valid 
// certificate authority.
https.createServer({

}, app).listen(app.get('port'), function() {
    console.log("Hello World.This is Airbot :)");
});
module.exports = app;

Link to github project : https://github.com/jw84/messenger-bot-tutorial

I am not not able to send the messages back to the user. Getting the following error :

Error sending messages:  { Error: socket hang up
     at TLSSocket.onHangUp (_tls_wrap.js:1124:19)
     at TLSSocket.g (events.js:291:16)
     at emitNone (events.js:91:20)
     at TLSSocket.emit (events.js:185:7)
     at endReadableNT (_stream_readable.js:974:12)
     at _combinedTickCallback (internal/process/next_tick.js:80:11)
     at process._tickDomainCallback (internal/process/next_tick.js:128:9) code: 'ECONNRESET' }

Appreciate help!

via FMBot Dev