[gnome-boxes] properties: Add a missing null check

[Zeeshan Ali Khattak <zeeshanak src gnome org>]

commit 07cee2b11b0f65615db201c822d0f8fd98a76a90
Author: Zeeshan Ali (Khattak) <zeeshanak gnome org>
Date:   Tue Aug 25 15:12:45 2015 +0100

    properties: Add a missing null check
    
    This avoids an annoying 'self != NULL' warning at runtime.

 src/properties.vala |    3 ++-
 1 files changed, 2 insertions(+), 1 deletions(-)
---
diff --git a/src/properties.vala b/src/properties.vala
index 25f203d..0982884 100644
--- a/src/properties.vala
+++ b/src/properties.vala
@@ -74,7 +74,8 @@ private class Boxes.Properties: Gtk.Notebook, Boxes.UI {
 
             for (var i = 0; i < PropertiesPage.LAST; i++) {
                 var page = get_data<PropertiesPageWidget> (@"boxes-property-$i");
-                reboot_required |= page.flush_changes ();
+                if (page != null)
+                    reboot_required |= page.flush_changes ();
             }
 
             var machine = window.current_item as Machine;