[gnome-shell] screenShield: fix trace back when unlocking session from another vt

[Ray Strode <halfline src gnome org>]

commit 4e8c4761530cc686c09c25ae3fda2dc4a024aa6c
Author: Ray Strode <rstrode redhat com>
Date:   Wed Oct 8 09:39:44 2014 -0400

    screenShield: fix trace back when unlocking session from another vt
    
    commit 1d374ac8bd496f6fa6f4c55ffd207bd30bd50075 introduced a bug that
    prevents unlock from working when initiated from another VT
    (user switching).
    
    This commit fixes the exception raised.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=708105

 js/ui/screenShield.js |    6 ++++--
 1 files changed, 4 insertions(+), 2 deletions(-)
---
diff --git a/js/ui/screenShield.js b/js/ui/screenShield.js
index 1c1dca6..32ef1ca 100644
--- a/js/ui/screenShield.js
+++ b/js/ui/screenShield.js
@@ -920,8 +920,10 @@ const ScreenShield = new Lang.Class({
         this._lockScreenState = MessageTray.State.HIDDEN;
         this._lockScreenGroup.hide();
 
-        this._dialog.actor.grab_key_focus();
-        this._dialog.actor.navigate_focus(null, Gtk.DirectionType.TAB_FORWARD, false);
+        if (this._dialog) {
+            this._dialog.actor.grab_key_focus();
+            this._dialog.actor.navigate_focus(null, Gtk.DirectionType.TAB_FORWARD, false);
+        }
     },
 
     _hideLockScreen: function(animate, velocity) {