How to use the @nebular/theme.NbSortDirection.NONE function in @nebular/theme

`,
  styleUrls: ['./tree-grid-shared.scss'],
})
export class TreeGridSortableComponent {
  customColumn = 'name';
  defaultColumns = [ 'size', 'kind', 'items' ];
  allColumns = [ this.customColumn, ...this.defaultColumns ];

  dataSource: NbTreeGridDataSource;

  sortColumn: string = '';
  sortDirection: NbSortDirection = NbSortDirection.NONE;

  constructor(private dataSourceBuilder: NbTreeGridDataSourceBuilder) {
    this.dataSource = this.dataSourceBuilder.create(this.data);
  }

  changeSort(sortRequest: NbSortRequest): void {
    this.dataSource.sort(sortRequest);
    this.sortColumn = sortRequest.column;
    this.sortDirection = sortRequest.direction;
  }

  getDirection(column: string): NbSortDirection {
    if (column === this.sortColumn) {
      return this.sortDirection;
    }
    return NbSortDirection.NONE;

}

@Component({
  selector: 'nb-tree-grid-showcase',
  templateUrl: './tree-grid-showcase.component.html',
  styleUrls: ['./tree-grid-shared.scss', './tree-grid-showcase.component.scss'],
})
export class TreeGridShowcaseComponent {
  customColumn = 'name';
  defaultColumns = [ 'size', 'kind', 'items' ];
  allColumns = [ this.customColumn, ...this.defaultColumns ];

  dataSource: NbTreeGridDataSource;

  sortColumn: string;
  sortDirection: NbSortDirection = NbSortDirection.NONE;

  constructor(private dataSourceBuilder: NbTreeGridDataSourceBuilder) {
    this.dataSource = this.dataSourceBuilder.create(this.data);
  }

  updateSort(sortRequest: NbSortRequest): void {
    this.sortColumn = sortRequest.column;
    this.sortDirection = sortRequest.direction;
  }

  getSortDirection(column: string): NbSortDirection {
    if (this.sortColumn === column) {
      return this.sortDirection;
    }
    return NbSortDirection.NONE;
  }