filepicker produces error with multiple and preview_images

There is a wierd situation which produces an error on load of the admin edit view:
* a filepicker field with `preview_images: true` and `multiple: true`
* with one file selected


Bonus: The preview image gets shown with one file selected, but not with multiple files selected.

I also can confirm it's not caused by number-only filenames (3.mpg) or files that are not images. This keeps reproducing with all-images content too.

I investigated this after it was [mentioned at discord](https://discord.com/channels/501836936584101899/506916956637495306/1067462603141546044) but i'm not smart enough to get my head around what's exactly happening in `themes/grav/app/forms/fields/filepicker.js`.

Here is an example blueprint snippet:
```yaml
header.gallery:
type: filepicker
label: Gallery
folder: 'self@'
preview_images: true
multiple: true
validate:
type: commalist
accept:
- jpg
- jpeg
- png
```