Crash after formatting with LVM and encryption

[Alfonso]

Previous install was Win8.1 and Ubuntu 17.10 without encryption.
Tried to install with erase disk, Ubuntu 18.04 only, with enceryption.
Ubiquity crashed soon after formatting.

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: Sat Apr 7 12:05:30 2018
InstallCmdLine: BOOT_IMAGE=/casper/vmlinuz.efi file=/cdrom/preseed/ubuntu.seed boot=casper quiet splash ---
LiveMediaBuild: Ubuntu 18.04 LTS "Bionic Beaver" - Beta amd64 (20180404)
SourcePackage: ubiquity
UpgradeStatus: No upgrade log present (probably fresh install)

[Alfonso (alfonso-martone)]

TLDR:
- if you press the "Back" key after partitioning, Ubiquity fails to repartition (Ubuntu 18.04-beta2), possibly bricking your PC.

This is definitely a problem because it bricked my Surface Pro 2:
- the error message only said there was an error, without any explanation;
- same error even after changing the formatting options (encryption, etc);
- after a reboot, Surface Pro 2 always shows its BIOS menu (which is limited to only two options: Secureboot and TPM) and won't boot Ubuntu USB installation anymore;
- the LED near the microSD port of the Surface will blink the sequence "3 short, 3 short, 3 short, 4 short, 1 long", meaning Firmware Panic "Secure Boot Loader loop"
- if you try to start the Ubuntu USB drive installation, it will blink "3 short, 4 short, 2 short, 1 short, 1 long" and won't boot anyway.

Why it happened:
- after starting installation and selecting Erase Disk + LVM encryption and clicking on Continue, I noticed I had Caps Lock still set; thus I clicked the "Go back" key when it became available again, wishing to restart the procedure using a password without all-caps; Ubiquity wouldn't be anymore able to go on.

Suggestion:
- repartitioning and formatting should only end when the partitions are in a clean state (for example including EFI files/structures required to boot).

Workaround: reinstall Windows from its "Recovery" USB drive (this will wipe the internal SSD and set up a clean EFI thing), then reinstall Ubuntu again:
- enable secure boot again from BIOS (power on while keeping VolumeDown pressed until the BIOS menu appears);
- reinstall Windows 8.1 from Recovery Drive (I created it on day one and kept that 8Gb USB drive in a safe place, including credentials for my never used Microsoft Account and my alternate email to receive the safe code at install time; the limited BIOS of the Surface Pro 2 will boot a Recovery Drive only if secure boot is enabled);
- boot Windows and go to PC Settings, select Alternate Boot menu (in the Update section of Settings) and choose USB drive boot;
- it won't boot Ubuntu USB drive at first (because it requires secure boot disabled); power off, then, while keeping the VolumeDown pressed, power on again and disable secure boot; then power off, insert Ubuntu install in the USB drive, keep VolumeDown pressed while powering on
- install Ubuntu as usual without going "Back" after partitioning.