Installer crash when following ManualFullSystemEncryption guide

[Patrick Fasano]

Encountered when following https://help.ubuntu.com/community/ManualFullSystemEncryption

> lsb_release -rd
Description: Ubuntu 18.04 LTS
Release: 18.04

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: ubiquity 18.04.14 [modified: lib/partman/automatically_partition/question]
ProcVersionSignature: Ubuntu 4.15.0-20.21-generic 4.15.17
Uname: Linux 4.15.0-20-generic x86_64
ApportVersion: 2.20.9-0ubuntu7
Architecture: amd64
Date: Mon May 14 21:30:13 2018
InstallCmdLine: BOOT_IMAGE=/multibootusb/ubuntu-mate-18.04-desktop-amd64/casper/vmlinuz file=/cdrom/preseed/ubuntu-mate.seed boot=casper iso-scan/filename= ignore_bootid live-media-path=/multibootusb/ubuntu-mate-18.04-desktop-amd64/casper cdrom-detect/try-usb=true floppy.allowed_drive_mask=0 ignore_uuid root=UUID=0C74-CB4A ---
SourcePackage: grub-installer
UpgradeStatus: No upgrade log present (probably fresh install)