Use Dispatcher to register MainWindow OnClosed event handler

Fixes WPF designer crashes when trying to expand the Miscellaneous properties.

Using debugger, we can learn that "Application.Current?.MainWindow" throws an exception as the calling thread cannot access MainWindow, because a different thread owns it.

Gu.Wpf.NumericInput/Touch/TextBox.cs

@@ -29,11 +29,14 @@ static TextBox()
                 typeof(System.Windows.Controls.TextBox),
                 UIElement.LostFocusEvent,
                 new RoutedEventHandler(OnLostFocus));
-            var mainWindow = Application.Current?.MainWindow;
-            if (mainWindow != null)
+            Application.Current.Dispatcher.Invoke(() =>
             {
-                mainWindow.Closed += OnClosed;
-            }
+                var mainWindow = Application.Current?.MainWindow;
+                if (mainWindow != null)
+                {
+                    mainWindow.Closed += OnClosed;
+                }
+            });
         }
 
         /// <summary>Helper for setting <see cref="ShowTouchKeyboardOnTouchEnterProperty"/> on <paramref name="element"/>.</summary>