[gnome-shell] overview: don't add a null desktop clone

[Giovanni Campagna <gcampagna src gnome org>]

commit 3fc478a14bed00ab746c55e3c98382957e105616
Author: Giovanni Campagna <gcampagna src gnome org>
Date:   Mon Feb 3 11:26:19 2014 +0100

    overview: don't add a null desktop clone
    
    If there is no desktop window, getDesktopClone() returns null.
    In that case, we know that no animation is needed.
    
    Fixes a regression from b97f3a9ecf52183e86506af622a08eaa7e3846bb
    
    https://bugzilla.gnome.org/show_bug.cgi?id=723523

 js/ui/overview.js |    9 +++++++--
 1 files changed, 7 insertions(+), 2 deletions(-)
---
diff --git a/js/ui/overview.js b/js/ui/overview.js
index 061175d..db52c54 100644
--- a/js/ui/overview.js
+++ b/js/ui/overview.js
@@ -493,8 +493,13 @@ const Overview = new Lang.Class({
     },
 
     fadeOutDesktop: function() {
-        if (!this._desktopFade.get_n_children())
-            this._desktopFade.add_child(this._getDesktopClone());
+        if (!this._desktopFade.get_n_children()) {
+            let clone = this._getDesktopClone();
+            if (!clone)
+                return;
+
+            this._desktopFade.add_child(clone);
+        }
 
         this._desktopFade.opacity = 255;
         this._desktopFade.show();