Bug 2486047

When using the Storage Editor to create and assign btrfs subvolumes, when clicking on "

Reproducible: Always

Steps to Reproduce:
1.Run installer. When at the Installation Method section, launch the storage editor from the three-dot menu.

2. In the Storage Editor, create a partition layout. I used GPT, created an EFI System Partition of about 1GB.

3. Create a root partition to be formatted in Btrfs. Do not directly assign a mount point to that root partition.

4. Create subvolumes in the partition. In my example I used:
@	/
@opt	/opt
@var.cache	/var/cache
@var.log	/var/log
@var.spool	/var/spool
@var.tmp	/var/tmp
@var.lib.containers	/var/lib/containers
@var.lib.flatpak	/var/lib/flatpak
@var.lib.libvirt	/var/lib/libvirt
@var.lib.plasmalogin	/var/lib/plasmalogin
@home	/home

5. Click "Return to installation". Wait for the configuration check to do its thing.

6. It will say "You have not defined a root partition (/)..." despite having selected the "@" subvolume to use as "/" mount point. Select "Proceed with installation".
Actual Results:
The installer goes right back to the Installation Method screen, rather than confirming the storage configuration and proceeding with the installation.

Expected Results:
The installer should proceed with the installation, confirming storage configuration first. It should also not warn about not having a root (/) partition as this is mounted from a btrfs subvolume.

Additional Information:
What is expected can be seen here: https://www.youtube.com/watch?v=d-CafjZf2M4
However unlike the above guide, using "@" and "." for clean separation of subdirectory subvolumes and for software such as Timeshift to work correctly. Plus at this stage it is more or less a de-facto convention for subvolumes.

Also in the Installation Method screen you can choose "Mount point assignment" and assign btrfs subvolumes that way, but that just is duplicating effort unnecessarily, is cumbersome and error prone. And the drop-down list for all the subvolumes often disappears off screen which makes it hard to select the right subvolume.