fix null reference in PostConfigureApplicationCookieTicketStore PostC…

…onfigure

src/IdentityServer/Configuration/DependencyInjection/PostConfigureApplicationCookieTicketStore.cs

@@ -61,7 +61,7 @@ public void PostConfigure(string name, CookieAuthenticationOptions options)
         {
             if(_httpContextAccessor.HttpContext == null)
             {
-                _logger.LogDebug("Failed to configure server side sessions for the authentication cookie scheme \"{scheme}\" because there is no current HTTP request");
+                _logger?.LogDebug("Failed to configure server side sessions for the authentication cookie scheme \"{scheme}\" because there is no current HTTP request");
                 return;
             }