Problem with using nodemailer to send email to user on Actions on Google, Dialogflow - actions-on-google

Question

I am using nodemailer to send emails to users, when an intent is triggered.
I am creating a promise for the function to send emails using nodemailer.
When running with node index.js, the emails are sent, but when deploying to firebase and invoking the intent with the simulator on actions on google console, i get a webhook error
MalformedResponse
Failed to parse Dialogflow response into AppResponse because of empty speech response.
Here is my code:
When the user invokes this intent, they should receive an email from me.
let sendMails = () => {
return new Promise((resolve, reject) => {
const transporter = nodemailer.createTransport({
service: 'gmail',
auth: {
user: 'my email',
pass: 'password'
}
});
let mailOptions = {
from: 'my email',
to: 'user's email',
subject: 'something',
text: 'here is the email'
};
transporter.sendMail(mailOptions, (error, info) => {
if (error) {
throw new Error("Error has come");
} else {
let answer = info.response;
return resolve(answer);
// console.log('Email sent: ' + info.response);
}
});
}).catch(error => {
console.log(error);
throw new Error("Error is coming")
})
}
And here is the intent:
app.intent('nda - yes', conv => {
return sendMails().then((answer)=> {
return conv.ask(`<speak>Done. Email is sent. Check your inbox.</speak>`)
})
})`