Bug 1478042 - Check if gBrowser exists in History menu for hidden tabs r=mak

MozReview-Commit-ID: F8zhbb4l7J8

--HG--
extra : rebase_source : 1a35e66c2bb8558ec76d84c2342eee695b910107

browser/base/content/browser-places.js

@@ -609,7 +609,8 @@ HistoryMenu.prototype = {
   },
 
   toggleHiddenTabs() {
-    if (gBrowser.visibleTabs.length < gBrowser.tabs.length) {
+    if (window.gBrowser &&
+        gBrowser.visibleTabs.length < gBrowser.tabs.length) {
       this.hiddenTabsMenu.removeAttribute("hidden");
     } else {
       this.hiddenTabsMenu.setAttribute("hidden", "true");