Crashes on attempt to install to btrfs-on-LUKS2 root

[Shreedhar Hegde]

Specific install scenario:
- Encrypted (LUKS2) btrfs volume (pre-formatted) set to be used as "/" in Ubiquity
- The concerned btrfs volume had a custom default subvolume (not FS_ROOT)
- A separate encrypted, pre-formatted physical swap partition was selected.
  On one occasion, Ubiquity crashed while trying to `mkswap` on this partition

To reproduce:
- On a VM (this happened on a VM), pass-through a USB 3.0 drive
- luksFormat a 40GB partition (LUKS2)
- Run mkfs.btrfs -K (I didn't want to send a bunch of TRIMs)
- Create a subvolume and set it as the default one (`btrfs subvolume set-default` was used)
- Create another 2GB LUKS2 partition, but this time run mkswap
- Ensure that the new swap partition is not being used (swapoff <new-mapped-part>
- Fire up Ubiquity
- Select these two partitions as "/" and swap space respectively. Ensure "format" is UNCHECKED
- "Install Now"
- Confirm partition map changes, as well as non-formatting of the "/" partition
- Sit back and wait for the crash (which you hopefully don't experience, but I have a bad feeling...)

ProblemType: Bug
DistroRelease: Ubuntu 20.04
Package: ubiquity 20.04.15.10
ProcVersionSignature: Ubuntu 5.8.0-43.49~20.04.1-generic 5.8.18
Uname: Linux 5.8.0-43-generic x86_64
NonfreeKernelModules: zfs zunicode zavl icp zcommon znvpair
ApportVersion: 2.20.11-0ubuntu27.16
Architecture: amd64
CasperMD5CheckResult: skip
CasperVersion: 1.445.1
Date: Thu Jul 1 02:58:14 2021
InstallCmdLine: BOOT_IMAGE=/casper/vmlinuz file=/cdrom/preseed/ubuntu.seed maybe-ubiquity quiet splash ---
LiveMediaBuild: Ubuntu 20.04.2.0 LTS "Focal Fossa" - Release amd64 (20210209.1)
SourcePackage: ubiquity
UpgradeStatus: No upgrade log present (probably fresh install)

[Shreedhar Hegde (mavenor)]

Of course, if somebody succeeds in replicating my little shenanigan... successfully, then any insights on what I'm doing wrong will be highly appreciated! :)
Until then, I guess I'll just `debootstrap' and piece stuff together myself...

[Shreedhar Hegde (mavenor)]

Oh, and I forgot to mention, quotas were DISABLED on the concerned btrfs volume, and the volume and all its subvolumes (I had a few) were unmounted when Ubiquity was started