Nextcloud Bookmark sync fails on Helium browser

### Which version of floccus are you using?

5.7.0

### How many bookmarks do you have, roughly?

329

### Are you using other means to sync bookmarks in parallel to floccus?

No

### Sync method

Nextcloud Bookmarks

### Which browser are you using? In case you are using the phone App, specify the Android or iOS version and device please.

Helium Version 0.5.8.1 (Official Build, Chromium 141.0.7390.122) (64-bit) on Windows 11

### Which version of Nextcloud Bookmarks are you using? (if relevant)

15.2.0

### Which version of Nextcloud? (if relevant)

31.0.8

### What kind of WebDAV server are you using? (if relevant)

_No response_

### Describe the Bug

After a successful initial sync on a fresh Helium installation, the automatic sync failed. The log reads: `Syncing failed with Cannot read properties of undefined (reading 'children')`. Subsequent manual syncs wouldn't start, or at least they generated no new logs. Completely removing Helium and reinstalling Helium and Floccus gave the same result.

### Expected Behavior

Since Helium is Chromium-based, I would expect Floccus to sync like it does in other Chromium-based browsers.

### To Reproduce

1. Install the latest version of Helium from their homepage, helium.computer.
2. Install Floccus Extension
3. Import a profile from another Chromium browser that uses Nextcloud Bookmarks as a storage backend.
4. Check the configuration to see if the bookmark path and URL look fine.
5. Start an initial sync.
6. Turn on auto-sync.

### Debug log provided

- [ ] I have provided a debug log file