Fix a race condition in the slide application

www/slide/main.js

@@ -32,7 +32,6 @@ define([
 
     module.spinner.show();
 
-    var ifrw = module.ifrw = $('#pad-iframe')[0].contentWindow;
     var stringify = function (obj) {
         return JSONSortify(obj);
     };
@@ -46,6 +45,7 @@ define([
     };
 
     $(function () {
+        var ifrw = module.ifrw = $('#pad-iframe')[0].contentWindow;
         var toolbar;
 
         var secret = Cryptpad.getSecrets();
@@ -126,7 +126,7 @@ define([
             var $modal = $pad.contents().find('#modal');
             var $content = $pad.contents().find('#content');
 
-            Slide.setModal($modal, $content, $pad);
+            Slide.setModal($modal, $content, $pad, ifrw);
 
             var enterPresentationMode = function (shouldLog) {
                 Slide.show(true, $textarea.val());

www/slide/slide.js

@@ -14,14 +14,15 @@ define([
         content: [],
         changeHandlers: [],
     };
-    var ifrw = $('#pad-iframe')[0].contentWindow;
+    var ifrw;
     var $modal;
     var $content;
     var $pad;
-    Slide.setModal = function ($m, $c, $p) {
+    Slide.setModal = function ($m, $c, $p, iframe) {
         $modal = Slide.$modal = $m;
         $content = Slide.$content = $c;
         $pad = Slide.$pad = $p;
+        ifrw = Slide.ifrw = iframe;
     };
 
     Slide.onChange = function (f) {