Fix all the readonlys!

Author: shana

src/GitHub.App/Factories/UIFactory.cs

@@ -43,10 +43,10 @@ public IUIPair CreateViewAndViewModel(UIViewType viewType)
     /// </summary>
     public class UIPair : IUIPair
     {
-        ExportLifetimeContext<IView> view;
-        ExportLifetimeContext<IViewModel> viewModel;
-        CompositeDisposable handlers = new CompositeDisposable();
-        UIViewType viewType;
+        readonly ExportLifetimeContext<IView> view;
+        readonly ExportLifetimeContext<IViewModel> viewModel;
+        readonly CompositeDisposable handlers = new CompositeDisposable();
+        readonly UIViewType viewType;
 
         public UIViewType ViewType => viewType;
         public IView View => view.Value;
@@ -88,9 +88,7 @@ void Dispose(bool disposing)
                 if (!handlers.IsDisposed)
                     handlers.Dispose();
                 view?.Dispose();
-                view = null;
                 viewModel?.Dispose();
-                viewModel = null;
                 disposed = true;
             }
         }

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

@@ -26,7 +26,7 @@ namespace GitHub.VisualStudio.UI.Views
     [PartCreationPolicy(CreationPolicy.NonShared)]
     public class GitHubPaneViewModel : TeamExplorerItemBase, IGitHubPaneViewModel
     {
-        CompositeDisposable disposables = new CompositeDisposable();
+        readonly CompositeDisposable disposables = new CompositeDisposable();
         IUIController uiController;
         WindowController windowController;