ItemsCountDisplayerExhibit.ts

// ...  
private listDataService: IListService;

  public onInit(): Promise<void> {
    return ComponentServices.init(this.context,
      this.properties, (startup, ctx, props) => {

        // // Register a new scoped instance of the service
        startup.registerScopedService(ListService.serviceKey, ListService);
        // Configure the service instance with the component specific properties
        startup.configureService(ListService.serviceKey, service => {
          service.configure(ctx.pageContext.web.absoluteUrl, props.listId);
        });
      
        // Must return a resolved promise 
        // (useless here but needed in case on async needs in the config process)
        return Promise.resolve();
      }).then(serviceScope => {

        // Consume the list service
        // Instead of keeping a service reference assigned here,
        // one can use ComponentServices.serviceScope.consume(ListService.serviceKey);
        this.listDataService = serviceScope.consume(ListService.serviceKey);
        // ...
      }).catch(error => {
        console.log('Error on init: ', error);
      });
  }

    // ...