Cannot Read Property 'top' occurs when click button to open modal window

The following code works when the page loads. However when I click on a button that is supposed to open a modal window I get the following error:

Uncaught TypeError: Cannot read property 'top' of undefined

How can I fix this error?

 if ($(window).width() > 992) {

  (function($) { "use strict";
    $(".page-scroll a[href^='#'], #intro a").on('click', function(e) {
       e.preventDefault();
       var hash = this.hash;
       $('html, body').stop().animate({
           scrollTop: $(hash).offset().top - 100}, 2000, 'easeOutExpo');
    });
  })(jQuery);

  $('body').scrollspy({
    target: '.navbar',
    offset: 110
  });
  // alert("large");
}
else {

  (function($) { "use strict";
    $(".page-scroll a[href^='#'], #intro a").on('click', function(e) {
       e.preventDefault();
       var hash = this.hash;
       $('html, body').stop().animate({
           scrollTop: $(hash).offset().top - 50}, 2000, 'easeOutExpo');
    });
  })(jQuery);

  $('body').scrollspy({
    target: '.navbar',
    offset: 70
  });
// alert("small");
}   

    //modal popup function
        function popup_modal(item){
            var link = $(item).attr('id');
      $('#bootstrap_modal').load('/'+link+'');
      $('#bootstrap_modal').modal('show'); 
      $("#bootstrap_modal").on("show", function () {
            $("body").addClass("modal-open");
            }).on("hidden", function () {
         $("body").removeClass("modal-open")
            });
        }

        //Modal pop up
        $('.mmodal').on('click', function(){
                popup_modal(this);
    });