How to make Reject comments mandatory in oracle workflow

Use Below Code

PROCEDURE btvl_reject_commnet_mandate (
      p_item_type   IN              VARCHAR2,
      p_item_key    IN              VARCHAR2,
      actid         IN              VARCHAR2,
      funmode       IN              VARCHAR2,
      resultout     OUT NOCOPY      VARCHAR2
   )
   IS
      v_response         VARCHAR2 (2000);
      l_user_comment     VARCHAR2 (2000);
      approval_comment   EXCEPTION;
   BEGIN
/*===================================================================================
This will derive the button by looking at the response from the notification. the WF_ENGINE.CONTEXT_NID variable will tell you the notification ID that is being responded to, and we can derive the response from there.
====================================================================================*/
      v_response :=
                wf_notification.getattrtext (wf_engine.context_nid, 'RESULT');
/*===================================================================================
This will retrieve the value which is stored in the Approval Comments field by the approver when the reject button is pressed.
====================================================================================*/
      l_user_comment :=
         wf_notification.getattrtext (wf_engine.context_nid,
                                      'COMMENTS'
                                     );

/*===================================================================================
This block will be fired when the user has pressed the 'Reject' Button and the comments is null and then it will raise the user defined error. And the workflow will not continue untill the user enters value in the comments field for rejection.
===================================================================================*/
      BEGIN
         IF (    funmode = 'RESPOND'
             AND v_response = 'REJECTED'
             AND l_user_comment IS NULL
            )
         THEN
            RAISE approval_comment;
         END IF;
      EXCEPTION
         WHEN approval_comment
         THEN
            raise_application_error
               (-20010,
                'Approval Comment Field cannot be Null. Please enter reason for rejection and click reject.'
               );
            RAISE;
      END;
   END;

After compiling above procedure then call this procedure to notification