Lubuntu Next Installer fails with volume encryption

[Artemgy]

Choosing "physical volume for encryption" makes the installer GUI fail, using Lubuntu Next artful Alpha 1 image:
http://cdimage.ubuntu.com/lubuntu-next/releases/artful/alpha-1/artful-desktop-amd64.iso

## Steps to reproduce

sudo swapoff -a # avoid bug with 'unsafe swap'
cat /proc/swaps
ubiquity &

* [Continue] through Welcome and Wireless to Disk
* choose "Manual" then [Continue]
    - NB: this is reinstalling over existing partitions from a previous install
* select swap partition - [Change] - "do not use parititon"
    - also to avoid bug with 'unsafe swap'
* boot partition - change - "Ext4" - check format - "/boot"
* main partition - change - "physical volume for encryption"
* click Install

This does NOTHING - the Install button just deselects itself when you click it!

## attempt to change options to make Install button work...

This might be a secondary issue.

* reselect main partition - [Change]

Installer Crashed

Traceback (most recent call last):
  File "/usr/lib/ubiquity/ubiquity/frontend/kde_components/PartMan.py", line 453, in on_edit_clicked
    self.partman_edit_dialog(devpart, partition)
  File "/usr/lib/ubiquity/ubiquity/frontend/kde_components/PartMan.py", line 367, in partman_edit_dialog
    self.ctrlr.dbfilter.edit_partition(devpart, **edits)
  File "/usr/lib/ubiquity/plugins/ubi-partman.py", line 3294, in edit_partition
    assert self.current_question == 'partman/choose_partition'
AssertionError

ProblemType: Bug
DistroRelease: Ubuntu 17.10
Package: ubiquity 17.04.9
ProcVersionSignature: Ubuntu 4.10.0-22.24-generic 4.10.15
Uname: Linux 4.10.0-22-generic x86_64
NonfreeKernelModules: wl
ApportVersion: 2.20.6-0ubuntu4
Architecture: amd64
CasperVersion: 1.383
CurrentDesktop: LXQt
Date: Sat Jul 22 18:09:08 2017
InstallCmdLine: BOOT_IMAGE=(hd0,4)/casper/vmlinuz.efi boot=casper quiet splash persistent --
LiveMediaBuild: Lubuntu-Next 17.10 "Artful Aardvark" - Alpha amd64 (20170628.1)
ProcEnviron:
 TERM=xterm
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=en_US.UTF-8
 SHELL=/bin/bash
SourcePackage: ubiquity
UpgradeStatus: No upgrade log present (probably fresh install)

[Artemgy (artemgy)]

md5sum artful-desktop-amd64.iso

58368fb72c9ae44bdea6a856375234ff artful-desktop-amd64.iso

[Artemgy (artemgy)]

Main issue is duplicate of https://bugs.launchpad.net/bugs/1510731
Secondary issue is a duplicate of https://bugs.launchpad.net/bugs/1510730

My workaround was to
1) manually create, format and open the physical volume for encryption
2) launch Ubiquity, and in Manual disk layout choose the /dev/mapper/sdaX_crypt volume as / (root) without formatting
3) complete the installation but choose Continue Testing
4) chroot into target, create crypttab and check fstab, update-initramfs

for step-by-step instructions see https://github.com/artmg/lubuild/blob/master/help/configure/LxQt-Kubuntu-Ubiqity-manual-encryption-bug.md

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.