Crash when selecting more than one crypto partitions

[Alessandro Grassi]

Trying to install kubuntu bionic beta2.

- Start installer
- Go to partitioning
- New partition table (optional)
- Create one partition, set "physical volume for encryption"
- Create another partition, set "physical volume for encryption"
- Crash

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: ubiquity 18.04.4
ProcVersionSignature: Ubuntu 4.15.0-13.14-generic 4.15.10
Uname: Linux 4.15.0-13-generic x86_64
ApportVersion: 2.20.9-0ubuntu2
Architecture: amd64
CasperVersion: 1.392
Date: Thu Apr 12 15:33:48 2018
InstallCmdLine: BOOT_IMAGE=///casper/vmlinuz.efi file=/cdrom/preseed/kubuntu.seed boot=casper maybe-ubiquity initrd=/casper/initrd.lz quiet splash ---
LiveMediaBuild: Kubuntu 18.04 LTS "Bionic Beaver" - Beta amd64 (20180404)
ProcEnviron:
 LANGUAGE=
 TERM=xterm-256color
 PATH=(custom, no user)
 LANG=C.UTF-8
 SHELL=/bin/bash
SourcePackage: ubiquity
UpgradeStatus: No upgrade log present (probably fresh install)