How to fix the selection of country if it did not select any country?

Just like this:

and it will remove all the data first inputed.

How to make the data still intact?
1. Go sitename.com/administrator/components/com_virtuemart/classes/
2. Open ps_userfield.php at line 696
3. Or find the code:

else if( formelement.options ) {

4. and after this code paste this code:

if(document.getElementById('country_field').value == ''){
                            alert('Country is required!');
                            return false;
                        } 

Then the bugs is fix.