UIController needs to be stopped when switching repos

Author: shana

src/GitHub.VisualStudio/UI/Views/GitHubPaneViewModel.cs

@@ -105,6 +105,7 @@ protected override void RepoChanged(bool changed)
             if (!changed)
                 return;
 
+            Stop();
             IsGitHubRepo = null;
             Reload().Forget();
         }