It happens because after fill the form, it is submitted. so when you try to get the input 'the first name', the form has been cleaned. In order to avoid this, just change the third parameter of casper.fill() method to false.
fill(String selector, Object values[, Boolean submit])
Example:
this.fill('form[name="phones_display"]', {
'first_name' : 'Joe', // Required
'last_name' : 'bloggs', // Required
....
}, false);