reinstall Ubuntu fails to reuse existing ESP partition

[Jean-Baptiste Lallement]

Test Case:
1. In UEFI mode perform a first installation of Ubuntu with the following partitioning scheme (adjust the size of the partition to your disk)
sda1 8G swap
sda2 10G /boot
sda3 100G /
sda4 100G /home
sda5 20G ESP

Proceed with the installation and reboot

2. Do a second installation and select to reinstall Ubuntu (and preserve existing data)
Continue with installation and reboot

Actual Result
The installer complains that it cannot find an existing ESP and asks the user to go back. However if you continue despite all the warnings the installer tries to perform the installation and at some point hangs completely without error. After the installation the existing system doesn't boot because the existing partitions have been wiped but not reinstalled (home has been preserved)

ProblemType: Bug
DistroRelease: Ubuntu 18.10
Package: ubiquity 18.10.12
ProcVersionSignature: Ubuntu 4.18.0-10.11-generic 4.18.12
Uname: Linux 4.18.0-10-generic x86_64
ApportVersion: 2.20.10-0ubuntu13
Architecture: amd64
CasperVersion: 1.399
Date: Wed Oct 17 11:46:27 2018
InstallCmdLine: BOOT_IMAGE=/casper/vmlinuz file=/cdrom/preseed/ubuntu.seed boot=casper only-ubiquity quiet splash ---
LiveMediaBuild: Ubuntu 18.10 "Cosmic Cuttlefish" - Release amd64 (20181017)
ProcEnviron:
 TERM=linux
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=C.UTF-8
 SHELL=/bin/bash
SourcePackage: ubiquity
UpgradeStatus: No upgrade log present (probably fresh install)

[Ubuntu QA Website (ubuntuqa)]

This bug has been reported on the Ubuntu ISO testing tracker.

A list of all reports related to this bug can be found here:
http://iso.qa.ubuntu.com/qatracker/reports/bugs/1798350