handle case when options.domElement is null

src/echo.js

@@ -30,7 +30,7 @@ window.Echo = (function (window, document, undefined) {
     offset = opts.offset || 0;
     throttle = opts.throttle || 250;
     domElement = opts.domElement ? document.querySelector(opts.domElement) : window;
-    var nodes = document.querySelectorAll( opts.domElement + ' [data-echo]');
+    var nodes = document.querySelectorAll( (opts.domElement || "") + ' [data-echo]');
 
     for (var i = 0; i < nodes.length; i++) {
       store.push(nodes[i]);