[chronojump] Fixed crash on delete_event on preferences window if thread is not defined

[Xavier de Blas <xaviblas src gnome org>]

commit aec7f4f0443bfcc15b63afaa53866b311b9f80f8
Author: Xavier de Blas <xaviblas gmail com>
Date:   Sun Jun 29 11:55:38 2014 +0200

    Fixed crash on delete_event on preferences window if thread is not defined

 src/gui/preferences.cs |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/src/gui/preferences.cs b/src/gui/preferences.cs
index 4529d4e..850f844 100644
--- a/src/gui/preferences.cs
+++ b/src/gui/preferences.cs
@@ -335,7 +335,7 @@ public class PreferencesWindow {
        void on_preferences_delete_event (object o, DeleteEventArgs args)
        {
                //do not hide/exit if copyiing
-               if (thread.IsAlive)
+               if (thread != null && thread.IsAlive)
                        args.RetVal = true;
                else {
                        PreferencesWindowBox.preferences.Hide();