Uncaught Error TypeError: Can not read property 'addEventListener' of null [duplicate]

I am using a modal window script, but it is causing the error and error Uncaught TypeError: Cannot read property 'addEventListener' of null and error: at Array.forEach (native)

Complete code:

( function( window ) {

'use strict';

// class helper functions from bonzo https://github.com/ded/bonzo

function classReg( className ) {   return new RegExp("(^|\s+)" + className + "(\s+|$)"); }

// classList support for class management // altho to be fair, the api sucks because it won't accept multiple classes at once var hasClass, addClass, removeClass;

if ( 'classList' in document.documentElement ) {   hasClass = function( elem, c ) {
    return elem.classList.contains( c );   };   addClass = function( elem, c ) {
    elem.classList.add( c );   };   removeClass = function( elem, c ) {
    elem.classList.remove( c );   }; } else {   hasClass = function( elem, c ) {
    return classReg( c ).test( elem.className );   };   addClass = function( elem, c ) {
    if ( !hasClass( elem, c ) ) {
      elem.className = elem.className + ' ' + c;
    }   };   removeClass = function( elem, c ) {
    elem.className = elem.className.replace( classReg( c ), ' ' );   }; }

function toggleClass( elem, c ) {   var fn = hasClass( elem, c ) ? removeClass : addClass;   fn( elem, c ); }

var classie = {   // full names   hasClass: hasClass,   addClass: addClass,   removeClass: removeClass,   toggleClass: toggleClass,   // short names   has: hasClass,   add: addClass,   remove: removeClass,   toggle: toggleClass };

// transport if ( typeof define === 'function' && define.amd ) {   // AMD   define( classie ); } else {   // browser global   window.classie
= classie; }

})( window );

/**  * modalEffects.js v1.0.0  * http://www.codrops.com  *  * Licensed under the MIT license.  * https://www.opensource.org/licenses/mit-license.php  *   * Copyright 2013, Codrops  * http://www.codrops.com  */ var ModalEffects = (function() {

    function init() {

        var overlay = document.querySelector( '.md-overlay' );

        [].slice.call( document.querySelectorAll( '.md-trigger' ) ).forEach( function( el, i ) {

            var modal = document.querySelector( '#' + el.getAttribute( 'data-modal' ) ),
                close = modal.querySelector( '.md-close' );

            function removeModal( hasPerspective ) {
                classie.remove( modal, 'md-show' );

                if( hasPerspective ) {
                    classie.remove( document.documentElement, 'md-perspective' );
                }           }

            function removeModalHandler() {
                removeModal( classie.has( el, 'md-setperspective' ) );              }

            el.addEventListener( 'click', function( ev ) {
                classie.add( modal, 'md-show' );
                overlay.removeEventListener( 'click', removeModalHandler );
                overlay.addEventListener( 'click', removeModalHandler );

                if( classie.has( el, 'md-setperspective' ) ) {
                    setTimeout( function() {
                        classie.add( document.documentElement, 'md-perspective' );
                    }, 25 );
                }           });

            close.addEventListener( 'click', function( ev ) {
                ev.stopPropagation();
                removeModalHandler();           });

        } );

    }

    init();

})();