'type: filepicker' not working inside plugin

EDIT: After digging around a bit more, and talking with the devs it looks like an issue with filepicker inside list elements on plugins. Added screenshots at the end.

I made a simple admin plugin addon that gets the blueprint of a page and builds a form so you can modify the frontmatter file of that page folder to help editors speed up multilanguage setups, but neither *type: pagemediaselect* or _type: filepicker_ work, giving me a prompt in admin saying that **'Cannot use "self@" outside pages'**.


The issue is I'm not using *self@* anywhere on my theme blueprints, neither on the plugin, the folder: is set to *theme://images/icons*.
```yaml
.image:
type: filepicker
folder: 'theme://images/icons'
preview_images: true
label: MYTHEME.FORM.MENUITEM_ICON
```
It is inside a list element. I placed another filepicker field outside of one to test, and it gave the same error, not working either.

On a clean install it doesn't work either, but doesn't throw an error.

I tried using it on the plugin blueprint itself, and it worked as expected

*type: mediapicker* does work but it only finds images uploaded to page folders, not the theme or *user://images*, which is what I need.

I've been using the latest version of both Grav, Forms and Admin plugin since the start.
I'm working on a windows 10 machine, and tried using both **bin/grav server** and xampp apache server.

EDIT:
Screenshots when adding elements with filepicker to a list inside the plugin
Adding new element
After saving it empty once it works normally
But every new element still doesn't work

Start by reproducing the issue in a clean Grav Admin setup with a filepicker inside a plugin list element, comparing it with the working plugin blueprint field. The payload names no source files or tests, so trace the Admin plugin's filepicker and list-element handling until theme:// or user:// selections work reliably; done means new and saved list elements no longer show the self@ error or empty picker.

Written by the indexing model from the issue text.