fix: CS8667 Nullability of reference types in type of

samples/ControlCatalog/Converter/HexConverter.cs

@@ -7,7 +7,7 @@ namespace ControlCatalog.Converter;
 
 public class HexConverter : IValueConverter
 {
-    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
+    public object? Convert(object? value, Type targetType, object? parameter, CultureInfo culture)
     {
         var str = value?.ToString();
         if (str == null)
@@ -18,7 +18,7 @@ public class HexConverter : IValueConverter
 
     }
 
-    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
+    public object? ConvertBack(object? value, Type targetType, object? parameter, CultureInfo culture)
     {
         try
         {