I need help debugging some Javascript from my WordPress site. I'm not a JS coder, so not sure what to do. I'm told there is an error on line 77: offset undefined. Can anyone help?

Code:
/* ==========================================================================
   Initiator Scripts
   ========================================================================== */

$(function() {
    // Initiate standard slider
    if ($('.flexslider').length > 0) {
        $('.flexslider').flexslider({
            animation: "slide",
            controlNav: false,
            directionNav: true,
            prevText: "<",
            nextText: ">",
            controlsContainer: ".flex-container"
        });
    }

    // Initiate Lifestyle page slider
    if ($('.location-slider').length > 0) {
        $('.location-slider').flexslider({
            animation: "slide",
            controlNav: true,
            directionNav: true,
            prevText: "<",
            nextText: ">",
            controlsContainer: ".location-slider-container"
        });

        // Append location description below link
        $('#location-slider .slide-1 a').css('background', 'url(../wp-content/themes/roots/assets/img/btn_slide1.jpg) top left');
        $('#location-slider .slide-2 a').css('background', 'url(../wp-content/themes/roots/assets/img/btn_slide2.jpg) top left');
        $('#location-slider .slide-3 a').css('background', 'url(../wp-content/themes/roots/assets/img/btn_slide3.jpg) top left');
        $('#location-slider .slide-4 a').css('background', 'url(../wp-content/themes/roots/assets/img/btn_slide4.jpg) top left');
        $('#location-slider .slide-5 a').css('background', 'url(../wp-content/themes/roots/assets/img/btn_slide5.jpg) top left');

        // Append location description below link
        $('#location-slider .slide-1').append('<span>Robina Town Centre</span>');
        $('#location-slider .slide-2').append('<span>Skilled Park Stadium</span>');
        $('#location-slider .slide-3').append('<span>Commercial Precinct</span>');
        $('#location-slider .slide-4').append('<span>Medical Precinct</span>');
        $('#location-slider .slide-5').append('<span>Transport Hub</span>');
    }

    // Initiate Fancybox popup
    if ($('.fancybox').length > 0) {
        $('.fancybox').fancybox();
    }
});



/* ==========================================================================
   Custom Scripts
   ========================================================================== */

// Mobile dropdown menu functionality
$(function() {
    $('.mobile-menu').on('click', function(){
        $('.nav-main ul').slideToggle();
    });
});


// Anchor links smooth scrolling
$(function($) {
    $(".scroll").click(function(event){
        event.preventDefault();
        $('html,body').animate({scrollTop:$(this.hash).offset().top}, 500);
    });
});


// Fixed sub-menu
var GblTop;

function GetVertOffset (srchStr) {
    GblTop = $(srchStr).offset().top - parseFloat($(srchStr).css('marginTop').replace(/auto/, 0));
}

$(function () {

    GetVertOffset ('.floating-sub-menu'); // -- Sets GblTop

    $(window).scroll (function() {
        // what is the y position of the scroll?
        var y = $(window).scrollTop();

        // whether that's below the start of article?
        if (y >= GblTop) {
            // if so, add the fixed class
            $('.floating-sub-menu').addClass('fixed-nav');
        } else {
            // otherwise, remove it
            $('.floating-sub-menu').removeClass('fixed-nav');
        }
    });
});