Authentication Ticket value is showing value as 'Nothing" While adding claims to it in OIDC integration of web forms application

I’ve integrated webforms application in to OIDC by using okta as authorization server. Everything works fine until getting Token response and User Info Response from OAuth server. when I try to add claims to the Authentication ticket of Authorized code received notification it was throwing error System.Null reference exception. I’ve put debug point and find Authentication ticket value showing as nothing. In Okta i have used grant type as ‘authorization code’ and in implementation i have used response type as ‘code’. Does some one explain why Authentication Ticket value shown as 'Nothing/Null? Also, what need to be done to get rid of this issue?