io.elementary.settings crashes with "free(): invalid pointer" on startup

### What Happened?

Settings crashes immediately on startup with "free(): invalid pointer" and aborted (core dumped). The following critical error appears in logs before the crash:

GLib-GObject-CRITICAL: value "-1" of type 'gint' is invalid or out of range for property 'position' of type 'gint'

The crash happens during wallpaper preview generation in the Desktop plug.

### Steps to Reproduce

1. Install elementary OS 8 (fresh install)
2. Open System Settings (io.elementary.settings)
3. Navigate to Desktop settings
4. Enable "Hide Dock when not being used"
5. Close and reopen System Settings — it crashes immediately on every subsequent launch

### Expected Behavior

Expected: Settings opens normally

### OS Version

8.x (Circe)

### OS Architecture

amd64 (on most hardwares)

### Session Type

Secure Session (Wayland, This is the default), Classic Session (X11)

### Software Version

Latest release (I have run all updates)

### Log Output

```shell
(io.elementary.settings:PID): GLib-GObject-CRITICAL **: value "-1" of type 'gint' is invalid or out of range for property 'position' of type 'gint'
free(): invalid pointer
Aborted (core dumped)
```

### Hardware Info

CPU: AMD Ryzen 5 5600 (6-core, 12-thread)
GPU: AMD Radeon RX 580 (amdgpu driver, Mesa 25.2.8)
RAM: 16 GiB
Motherboard: ASUS PRIME B550M-K
Display: Dual monitor (1440x900 + 1920x1080), X11
Kernel: 6.17.0-19-generic x86_64
io.elementary.settings.desktop 8.2.1+r2288+pkg928~ubuntu8.1