Calling Repository Pattern methods returns null in Asp.Net Mvc Core

Hello i am using in an Asp.net MVC CORE project a repository pattern adapted from here. while i am using the GetAll() method to make a call to the database works if i try to use

T GetSingle(object id);
T GetSingle(Expression<Func<T, bool>> predicate);

like this var model = _repository.GetSingle(x => x.Id == id);

or like this var model = _repository.GetSingle(id);

Model returns Null.

in the Razor View if i change from @model IEnumerable<ApplicationUser> which works ok with the GetAll()

to this @model ApplicationUser i get Null. My controller is like this:

[HttpGet]
public IActionResult Index(string id)
{
    //var model = _repository.GetAll();
    var model = _repository.GetSingle(x => x.Id == id);
    //var model = _repository.GetSingle(id);
    if(model == null)
    {
        return NotFound();
    }
    return View(model);
}

Here is the declaration of the Repository if it helps:

public class EntityBaseRepository<T> : IRepository<T> where T : class, new() 

i am wondering why database returns Null with those two methods??

these are the methods

        public T GetSingle(object id)
        {
            return _context.Set<T>().Find(id);
        }
        public T GetSingle(Expression<Func<T, bool>> predicate)
        {
            return _context.Set<T>().FirstOrDefault(predicate);
        }
            public IEnumerable<T> GetAll()
        {
            return _context.Set<T>().AsEnumerable();
        }