Return EnumValues for nullable enum.

[maliming]

The temporary solution is to replace the CachedObjectExtensionsDtoService:

using System;
using System.Collections.Generic;
using System.Linq;
using Volo.Abp.AspNetCore.Mvc.ApplicationConfigurations.ObjectExtending;
using Volo.Abp.DependencyInjection;
using Volo.Abp.ObjectExtending;
using Volo.Abp.ObjectExtending.Modularity;

namespace MyCompanyName.MyProjectName.Web;

[Dependency(ReplaceServices = true)]
[ExposeServices(typeof(MyCachedObjectExtensionsDtoService), typeof(CachedObjectExtensionsDtoService), typeof(ICachedObjectExtensionsDtoService))]
public class MyCachedObjectExtensionsDtoService : CachedObjectExtensionsDtoService
{
    public MyCachedObjectExtensionsDtoService(IExtensionPropertyAttributeDtoFactory extensionPropertyAttributeDtoFactory)
        : base(extensionPropertyAttributeDtoFactory)
    {

    }

    protected override void FillEnums(ObjectExtensionsDto objectExtensionsDto)
    {
        var enumProperties = ObjectExtensionManager.Instance.Modules().Values
            .SelectMany(
                m => m.Entities.Values.SelectMany(
                    e => e.GetProperties()
                )
            )
            .Where(p => p.Type.IsEnum || IsNullableEnum(p.Type))
            .ToList();

        foreach (var enumProperty in enumProperties)
        {
            // ReSharper disable once AssignNullToNotNullAttribute (enumProperty.Type.FullName can not be null for this case)
            objectExtensionsDto.Enums[enumProperty.Type.FullName!] = CreateExtensionEnumDto(enumProperty);
        }
    }

    protected override ExtensionEnumDto CreateExtensionEnumDto(ExtensionPropertyConfiguration enumProperty)
    {
        var extensionEnumDto = new ExtensionEnumDto
        {
            Fields = new List<ExtensionEnumFieldDto>(),
            LocalizationResource = enumProperty.GetLocalizationResourceNameOrNull()
        };

        var enumType = enumProperty.Type.IsEnum
            ? enumProperty.Type
            : 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 = enumType.GetEnumName(enumValue)!,
                    Value = enumValue
                }
            );
        }

        return extensionEnumDto;
    }

    public bool IsNullableEnum(Type type)
    {
        return type.IsGenericType &&
               type.GetGenericTypeDefinition() == typeof(Nullable<>) &&
               type.GenericTypeArguments.Length == 1 &&
               type.GenericTypeArguments[0].IsEnum;
    }
}