message: fix: stringify group label
  closes
diff: @@ -370,7 +370,7 @@ export class ItemsList {
  }
  const isObjectKey = isObject(key);
  const parent: NgOption = {
  - label: isObjectKey ? '' : <string>key,
  + label: isObjectKey ? '' : String(key),
  children: undefined,
  parent: null,
  index: i++,
  