Fix compile error

Author: Lolle2000la

src/ImageSort.WPF/App.xaml.cs

@@ -43,21 +43,20 @@ public App()
 #endif
 
         RxAppBuilder.CreateReactiveUIBuilder()
+            .WithExceptionHandler(ex =>
+            {
+                Application.Current?.Dispatcher.Invoke(() =>
+                {
+                    MessageBox.Show(
+                        $"ReactiveUI Pipeline Exception:\n\n{ex.Message}\n\nStack Trace:\n{ex.StackTrace}",
+                        "ReactiveUI Error",
+                        MessageBoxButton.OK,
+                        MessageBoxImage.Error);
+                });
+            })
             .WithCoreServices()
             .BuildApp();
 
-        RxApp.DefaultExceptionHandler = Observer.Create<Exception>(ex =>
-        {
-            Application.Current?.Dispatcher.Invoke(() =>
-            {
-                MessageBox.Show(
-                    $"ReactiveUI Pipeline Exception:\n\n{ex.Message}\n\nStack Trace:\n{ex.StackTrace}",
-                    "ReactiveUI Error",
-                    MessageBoxButton.OK,
-                    MessageBoxImage.Error);
-            });
-        });
-
         Locator.CurrentMutable.RegisterViewsForViewModels(Assembly.GetEntryAssembly());
         Locator.CurrentMutable.RegisterManditoryDependencies();
         Locator.CurrentMutable.Register<IRecycleBin>(() => new RecycleBin());