screenShield: Don't crash when trying to deactivate the shield

If the user has a lock delay, or deactivate() has been called at
any other time, we need to check for the unlock dialog, as it may
not always exist.

js/ui/screenShield.js

@@ -1123,9 +1123,12 @@ const ScreenShield = new Lang.Class({
     },
 
     deactivate: function(animate) {
+        if (this._dialog)
             this._dialog.finish(Lang.bind(this, function() {
                 this._finishDeactivate(animate);
             }));
+        else
+            this._finishDeactivate(animate);
     },
 
     _finishDeactivate: function(animate) {