fix isTreeItem method when called with non-element nodes (#1026)

src/components/tree-item/tree-item.ts

@@ -14,8 +14,8 @@ import '../spinner/spinner';
 import styles from './tree-item.styles';
 import type { CSSResultGroup, PropertyValueMap } from 'lit';
 
-export function isTreeItem(element: Element) {
-  return element && element?.getAttribute('role') === 'treeitem';
+export function isTreeItem(node: Node) {
+  return node instanceof Element && node.getAttribute('role') === 'treeitem';
 }
 
 /**