Return `EnumValues` for `nullable enum`.

framework/src/Volo.Abp.AspNetCore.Mvc/Volo/Abp/AspNetCore/Mvc/ApplicationConfigurations/ObjectExtending/CachedObjectExtensionsDtoService.cs

@@ -242,7 +242,7 @@ public class CachedObjectExtensionsDtoService : ICachedObjectExtensionsDtoServic
                     e => e.GetProperties()
                 )
             )
-            .Where(p => p.Type.IsEnum)
+            .Where(p => p.Type.IsEnum || TypeHelper.IsNullableEnum(p.Type))
             .ToList();
 
         foreach (var enumProperty in enumProperties)
@@ -260,12 +260,23 @@ public class CachedObjectExtensionsDtoService : ICachedObjectExtensionsDtoServic
             LocalizationResource = enumProperty.GetLocalizationResourceNameOrNull()
         };
 
-        foreach (var enumValue in enumProperty.Type.GetEnumValues())
+        var enumType = enumProperty.Type.IsEnum
+            ? enumProperty.Type
+            : TypeHelper.IsNullableEnum(enumProperty.Type)
+                ? Nullable.GetUnderlyingType(enumProperty.Type)
+                : null;
+
+        if (enumType == null)
+        {
+            return extensionEnumDto;
+        }
+
+        foreach (var enumValue in enumType.GetEnumValues())
         {
             extensionEnumDto.Fields.Add(
                 new ExtensionEnumFieldDto
                 {
-                    Name = enumProperty.Type.GetEnumName(enumValue)!,
+                    Name = enumType.GetEnumName(enumValue)!,
                     Value = enumValue
                 }
             );

framework/src/Volo.Abp.Core/Volo/Abp/Reflection/TypeHelper.cs

@@ -84,6 +84,14 @@ public static class TypeHelper
         return type.IsGenericType && type.GetGenericTypeDefinition() == typeof(Nullable<>);
     }
 
+    public static bool IsNullableEnum(Type type)
+    {
+        return type.IsGenericType &&
+               type.GetGenericTypeDefinition() == typeof(Nullable<>) &&
+               type.GenericTypeArguments.Length == 1 &&
+               type.GenericTypeArguments[0].IsEnum;
+    }
+
     public static Type GetFirstGenericArgumentIfNullable(this Type t)
     {
         if (t.GetGenericArguments().Length > 0 && t.GetGenericTypeDefinition() == typeof(Nullable<>))