src/vs/workbench/api/common/extHostTreeViews.ts/ExtHostTreeView/validateTreeItem
class ExtHostTreeView:	private validateTreeItem(extensionTreeItem: vscode.TreeItem) {
		if (!TreeItem.isTreeItem(extensionTreeItem, this.extension)) {
			throw new Error(`Extension ${this.extension.identifier.value} has provided an invalid tree item.`);
		}
	}