1. This is a typo. It should be return this.widgetsQuery.isLoading(id);
2. You are right. This part is missing. You should have a query method:

export class WidgetsQuery extends QueryEntity<WidgetsState, Widget> {
  ui: EntityUIQuery<WidgetsUIState, WidgetUI>;
constructor( protected store: WidgetsStore ) {
    super(store);
    this.createUIQuery();
  }
  
  selectLoadingEntity(id: ID): Observable<boolean> {
    return this.ui.selectEntity(id, 'isLoading');
  }
}

Updated the gists.