Fix exception when hooking repochanged event

Check IVSGitExt isn't null before hooking ActiveRepositoriesChanged event.

Author: jcansdale

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

@@ -35,7 +35,10 @@ public TeamExplorerServiceHolder(IVSGitExt gitService)
             syncContext = SynchronizationContext.Current;
 
             UpdateActiveRepo();
-            gitService.ActiveRepositoriesChanged += UpdateActiveRepo;
+            if (gitService != null)
+            {
+                gitService.ActiveRepositoriesChanged += UpdateActiveRepo;
+            }
         }