code: @PostConstruct
      private void configureMessagesExceptionArgumentResolvers() {
          this.handlerExceptionResolver = this.applicationContext.getBean("handlerExceptionResolver", HandlerExceptionResolver.class);
  
          for (final HandlerExceptionResolver resolver : ((HandlerExceptionResolverComposite) this.handlerExceptionResolver).getExceptionResolvers()) {
              if (resolver instanceof ExceptionHandlerExceptionResolver) {
                  configureCustomHandlerMethodArgumentResolver((ExceptionHandlerExceptionResolver) resolver);
              }
          }
      }
docs: adds a FlashMessagesMethodArgumentResolver to handlerExceptionResolver's argument resolvers