[gitg] Added missing check for null repository

[Jesse van den Kieboom <jessevdk src gnome org>]

commit 406baa8ce4a7f14b678b43dbf11445f4f4d8bed4
Author: Jesse van den Kieboom <jessevdk gnome org>
Date:   Sat Jul 12 11:18:26 2014 +0200

    Added missing check for null repository

 gitg/history/gitg-history-refs-list.vala |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/gitg/history/gitg-history-refs-list.vala b/gitg/history/gitg-history-refs-list.vala
index 6ad114f..ff60a4d 100644
--- a/gitg/history/gitg-history-refs-list.vala
+++ b/gitg/history/gitg-history-refs-list.vala
@@ -755,7 +755,7 @@ public class RefsList : Gtk.ListBox
 
                        try
                        {
-                               if (d_repository.is_head_detached())
+                               if (d_repository != null && d_repository.is_head_detached())
                                {
                                        ret.add(d_repository.get_head());
                                }