Generating token and expiration with JWT SecurityTokenDescriptor on c#

When I try to create token and expiration SecurityTokenDescriptor for a syntax it doesn't generate an error but when I call this function on postman it gives that error "System.NullReferenceException: Object reference not set to an instance of an object." My code below here.

[HttpPost]
    [Route("Login")]
    public async Task<ActionResult<KullaniciModel>> Authenticate(KullaniciLoginDTO kullaniciLoginDTO)
    { 
        KullaniciModel kullaniciModel = new KullaniciModel();

        var user = await _context.KullaniciModel.SingleOrDefaultAsync(x=> x.kullaniciEPosta == kullaniciLoginDTO.kullaniciEPosta);
        if(HashingHelper.VerifyPassword(kullaniciLoginDTO.sifre,  user.passwordhash,  user.passwordsalt )) // sifre means password
        {
            var tokenDescriptor = new SecurityTokenDescriptor
            {
                Subject = new ClaimsIdentity(new Claim[]
                {
                    new Claim("kullaniciId", user.kullaniciId.ToString()), //kullaniciId means userID
                    new Claim("kullaniciRol", user.kullaniciRol.ToString()),
                }),
                Expires = DateTime.UtcNow.AddMinutes(75),
                SigningCredentials = new SigningCredentials(new SymmetricSecurityKey(Encoding.UTF8.GetBytes(_appSettings.JWT_Secret)), SecurityAlgorithms.HmacSha256Signature)
            };
            
            var tokenHandler = new JwtSecurityTokenHandler();
            var securityToken = tokenHandler.CreateToken(tokenDescriptor);
            var token = tokenHandler.WriteToken(securityToken);


            return Ok(new { token });

        }
        else
        {
            return BadRequest(new { message = "Kullanıcı Girişi Yapılamadı" });
        }
    }

At this part "var tokenDescriptor = new SecurityTokenDescriptor" generates an error that is shown above.

What was the problem I couldn't find any solution to that