src/vs/base/browser/ui/tree/asyncDataTree.ts/AsyncDataTree/getFirstElementChild
class AsyncDataTree:	getFirstElementChild(element: TInput | T = this.root.element): TInput | T | undefined {
		const dataNode = this.getDataNode(element);
		const node = this.tree.getFirstElementChild(dataNode === this.root ? null : dataNode);
		return (node && node.element)!;
	}