Javascript error - cannot call method 'appendChild' of null (svgicons)

I get this error in the Chrome Developer Tools, but I can't see any errors visually on the page.

Here is a short extract of the JS:

function svgIcon( el, config, options ) {
        this.el = el;
        this.options = extend( {}, this.options );
        extend( this.options, options );
        this.svg = Snap( this.options.size.w, this.options.size.h );
        this.svg.attr( 'viewBox', '0 0 64 64' );
        this.el.appendChild( this.svg.node );
        // state
        this.toggled = false;
        // click event (if mobile use touchstart)
        this.clickevent = mobilecheck() ? 'touchstart' : 'click';
        // icons configuration
        this.config = config[ this.el.getAttribute( 'data-icon-name' ) ];
        // reverse?
        if( hasClass( this.el, 'si-icon-reverse' ) ) {
            this.reverse = true;
        }
        if( !this.config ) return;
        var self = this;

I have tried solving the problem by looking into other similar cases on Stackoverflow, and it points out that there are either something missing or something with the order it is read/written in.

I have not written svgicons.js, so there are not supposed to be any issues with it. However I can see that some has the same issues as me, but there are not provideded an explanation or answer for the problem. Here is basically the same question from another person.

Check your hamburger_animations.js file - it's at http://www.vekvemedia.no/wp-content/themes/vekvemedia/js/hamburger_animation.js

(function() {                           
    [].slice.call( document.querySelectorAll( '.si-icons-default > .si-icon' ) ).forEach( function( el ) {
        var svgicon = new svgIcon( el, svgIconConfig );
    } );
    new svgIcon( document.querySelector( '.si-icons-easing .si-icon-hamburger' ), svgIconConfig, { easing : mina.backin } );
    new svgIcon( document.querySelector( '.si-icons-easing .si-icon-hamburger-cross' ), svgIconConfig, { easing : mina.elastic, speed: 600              } );
    })();

document.querySelector( '.si-icons-easing .si-icon-hamburger' ) is resulting in null