Use xunit with unitofwork and repository in dotnet 6

public interface IUnitOfWork : IDisposable
{
    IUserRepository UserRepository { get; }
}

public class UnitOfWork : IUnitOfWork
{
    protected readonly DatabaseContext db;

    private IUserRepository userRepository;
    public IUserRepository UserRepository
    {
        get
        {
            if (userRepository == null)
            {
                userRepository = new UserRepository(db);
            }

            return userRepository;
        }
    }

    //...
}

public interface IUserRepository : IGenericRepository<UserRole>
{
    Task<User> GetUserWithRolesAsync(string username);
}

public class UserRepository : GenericRepository<User>, IUserRepository
{
    public async Task<User> GetUserWithRolesAsync(string username)
    {
        //...
    }
}

private readonly IUnitOfWork _uow;
public AuthController(IUnitOfWork uow) : base(uow)
{
    _uow = uow;
}

[HttpPost("authenticate")]
public async Task<IActionResult> Authenticate([FromBody] UserLoginViewModel Request)
{
    User user = await _uow.UserRepository.GetUserWithRolesAsync(Request.Username);
    //....
}

[Fact]
public async Task Authenticate_WithInvalidUsernamePassword_ReturnsNotFound()
{
    // Arrange
    var unitOfWorkStub = new Mock<IUnitOfWork>();
    unitOfWorkStub.Setup(uow => uow.UserRepository.GetUserWithRolesAsync(It.IsAny<string>())).ReturnsAsync((User)null);

    var jwtAuthenticatorManagerStub = new Mock<IJwtAuthenticatorManager>();
    var localizorStub = new Mock<IStringLocalizer<SharedTranslate>>();

    var controller = new AuthController(unitOfWorkStub.Object, localizorStub.Object, jwtAuthenticatorManagerStub.Object);
    UserLoginViewModel userLoginViewModel = new UserLoginViewModel
    {
        Username = "admin",
        Password = "admin@123"
    };

    // Act
    var result = await controller.Authenticate(userLoginViewModel);

    // Assert
    Assert.IsType<NotFoundResult>(result);
}