[gnome-shell] Fix regression from 38f943ef81781210a7b680ecd58b75cb9007635a

[Giovanni Campagna <gcampagna src gnome org>]

commit 633bbd8a9e28d86c89a081fd697cc2b96f039b24
Author: Giovanni Campagna <gcampagna src gnome org>
Date:   Sun Sep 16 22:36:21 2012 +0200

    Fix regression from 38f943ef81781210a7b680ecd58b75cb9007635a
    
    In gdm mode there is no activities button, and accessing it causes an
    exception.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=684162

 js/ui/messageTray.js |    3 ++-
 1 files changed, 2 insertions(+), 1 deletions(-)
---
diff --git a/js/ui/messageTray.js b/js/ui/messageTray.js
index cfbf011..f11aa5b 100644
--- a/js/ui/messageTray.js
+++ b/js/ui/messageTray.js
@@ -1443,7 +1443,8 @@ const MessageTray = new Lang.Class({
         this._grabHelper = new GrabHelper.GrabHelper(this.actor);
         this._grabHelper.addActor(this._summaryBoxPointer.actor);
         this._grabHelper.addActor(this.actor);
-        this._grabHelper.addActor(Main.panel.statusArea['activities'].hotCorner.actor);
+        if (Main.panel.statusArea.activities)
+            this._grabHelper.addActor(Main.panel.statusArea.activities.hotCorner.actor);
 
         Main.layoutManager.keyboardBox.connect('notify::hover', Lang.bind(this, this._onKeyboardHoverChanged));