src/vs/workbench/api/common/extHostTreeViews.ts/ExtHostTreeView/createAndRegisterTreeNode
class ExtHostTreeView:	private createAndRegisterTreeNode(element: T, extTreeItem: vscode.TreeItem, parentNode: TreeNode | Root): TreeNode {
		const node = this.createTreeNode(element, extTreeItem, parentNode);
		if (extTreeItem.id && this.elements.has(node.item.handle)) {
			throw new Error(localize('treeView.duplicateElement', 'Element with id {0} is already registered', extTreeItem.id));
		}
		this.addNodeToCache(element, node);
		this.addNodeToParentCache(node, parentNode);
		return node;
	}