White page on custom plugin option submit

Your button is submitting the form
use this script

<script>
    jQuery(document).ready(function ($) {
        $("#add-url").click(function(e){
           e.preventDefault();
            $( "#submit" ).before( "<br  /><input id='url-field-1' name="urlfield[]" type="text"><br  />" );
  });
    });
    </script>