after adding obj{} inside For-look now two values getting populated 

fields.push({
type: 'choice',
name: 'invoice_type',
label: getMessage('invoice type'),
choices: answer.choices,
mandatory: true
});

I request you kindly update your client script and script-include with your latest changes in thread , i will mark it. 

Can't we show "-- None --" ? and user must select value