Why Context Return Null in entity framework

Question

why validate return null although i have it

public Registration ValidateUser(string userName, string passWord)
{
try
{
using (var _context = new DatabaseContext())
{
var validate = (from user in _context.Registration
where user.Username == userName && user.Password == passWord
select user).SingleOrDefault();
var users=_context.Registration;
return validate;
}
}
catch (Exception)
{
throw;
}
}

public DbSet<Registration> Registration { get; set; }

and in db i have Registration table with records