Prevents null value.

Terminal.Gui/Windows/MessageBox.cs

@@ -290,8 +290,8 @@ namespace Terminal.Gui {
 
 			if (useErrorColors) {
 				d.ColorScheme = Colors.Error;
-				border.BorderBrush = Colors.Error.Normal.Foreground;
-				border.Background = Colors.Error.Normal.Background;
+				d.Border.BorderBrush = Colors.Error.Normal.Foreground;
+				d.Border.Background = Colors.Error.Normal.Background;
 			} else {
 				d.ColorScheme = Colors.Dialog;
 				d.Border.BorderBrush = Colors.Dialog.Normal.Foreground;