load the session if the session is not preloaded already

app/controllers/doorkeeper/tokens_controller.rb

@@ -6,6 +6,7 @@ class TokensController < Doorkeeper::ApplicationMetalController
 
     def create
       headers.merge!(authorize_response.headers)
+      session.send(:load!) if session.id.nil?
       render json: authorize_response.body,
              status: authorize_response.status
     rescue Errors::DoorkeeperError => e