$(document).on('show.bs.modal', '.modal', function (event) {
  var zIndex = 1040 + (10 * $('.modal:visible').length);
  $(this).css('z-index', zIndex);
  setTimeout(function() {
    $('.modal-backdrop').not('.modal-stack').css('z-index', zIndex - 1).addClass('modal-stack');
  }, 0);
});