Fix null ref exception in SafeMode

Author: jcansdale

src/GitHub.TeamFoundation.14/Base/TeamExplorerServiceHolder.cs

@@ -135,7 +135,9 @@ void NotifyActiveRepo()
 
         void UpdateActiveRepo()
         {
-            var repo = gitService.ActiveRepositories.FirstOrDefault();
+            // NOTE: gitService will be null in Expression Blend or Safe Mode
+            var repo = gitService?.ActiveRepositories.FirstOrDefault();
+
             if (!Equals(repo, ActiveRepo))
                 // so annoying that this is on the wrong thread
                 syncContext.Post(r => ActiveRepo = r as ILocalRepositoryModel, repo);