parentPath can never be null. It will be handled by the condition above

src/Umbraco.Web.UI.Client/src/packages/core/tree/file-system-tree.store.ts

@@ -38,7 +38,7 @@ export class UmbFileSystemTreeStore
 		return this._data.asObservablePart((items) =>
 			items.filter((item) => {
 				const pathCut = item.path?.substring(0, item.path?.lastIndexOf('/'));
-				return parentPath === pathCut || parentPath === null;
+				return parentPath === pathCut;
 			}),
 		);
 	}