src/vs/workbench/api/common/extHostTreeViews.ts/ExtHostTreeView/getChildren
class ExtHostTreeView:	async getChildren(parentHandle: TreeItemHandle | Root): Promise<ITreeItem[] | undefined> {
		const parentElement = parentHandle ? this.getExtensionElement(parentHandle) : undefined;
		if (parentHandle && !parentElement) {
			this.logService.error(`No tree item with id \'${parentHandle}\' found.`);
			return Promise.resolve([]);
		}

		let childrenNodes: TreeNode[] | undefined = this.getChildrenNodes(parentHandle); // Get it from cache

		if (!childrenNodes) {
			childrenNodes = await this.fetchChildrenNodes(parentElement);
		}

		return childrenNodes ? childrenNodes.map(n => n.item) : undefined;
	}