User.Identity.Name returns null in ASP.NET Core MVC application

Please check the code below. This is a controller of my ASP.NET Core 2.1 MVC application.

Problem is that I am already using cookie authenticated user, but the User.Identity.Name returns null. How can I get my current auth user on .NET Core then?

[HttpPost]
public IActionResult AddMainMenu(MainMenuItems mainMenuItems)
{
      string userEmail = User.Identity.Name;
      return Json("success");          
}

Thanks in advance.