[gnome-boxes] Fix set_ui_state() warning

[Marc-Andre Lureau <malureau src gnome org>]

commit cbfd3b02da347fbcc84cc9b3e83f7952416cfb5f
Author: Marc-Andrà Lureau <marcandre lureau gmail com>
Date:   Wed Nov 2 17:06:40 2011 +0100

    Fix set_ui_state() warning

 src/collection-view.vala |    3 ++-
 1 files changed, 2 insertions(+), 1 deletions(-)
---
diff --git a/src/collection-view.vala b/src/collection-view.vala
index 1560e33..a1834d7 100644
--- a/src/collection-view.vala
+++ b/src/collection-view.vala
@@ -90,7 +90,8 @@ private class Boxes.CollectionView: Boxes.UI {
             over_boxes.pack (ui_state == UIState.WIZARD ? app.wizard.actor : app.properties.actor);
             app.wizard.actor.add_constraint (new Clutter.BindConstraint (over_boxes, BindCoordinate.SIZE, 0));
             actor_add (over_boxes, app.stage);
-            app.current_item.ui_state = ui_state;
+            if (app.current_item != null)
+                app.current_item.ui_state = ui_state;
             break;
 
         default: