Crash when trying to format existing ext4 partition

[Nikolay Pliashechnikov]

I installed Kubuntu 18.04 to a new SSD. Then, after effectively destroying it, I decided to make a clean reinstall, so I booted from a LiveUSB, connected to the network and launched the installer.
I decided not to repartition the SSD, but rather to delete and re-create existing ext4 partition. After recreating it, I tried to click on "Format" checkbox and installer immediately crashed with the following stack trace:

Traceback (most recent call last):
  File "/usr/lib/ubiquity/ubiquity/frontend/kde_components/PartitionModel.py", line 66, in setData
    item.partman_column_format_toggled(value.toBool())
AttributeError: 'int' object has no attribute 'toBool'

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: ubiquity 18.04.14
ProcVersionSignature: Ubuntu 4.15.0-20.21-generic 4.15.17
Uname: Linux 4.15.0-20-generic x86_64
NonfreeKernelModules: wl
ApportVersion: 2.20.9-0ubuntu7
Architecture: amd64
CasperVersion: 1.394
CurrentDesktop: KDE
Date: Thu May 24 10:50:19 2018
InstallCmdLine: file=/cdrom/preseed/kubuntu.seed boot=casper maybe-ubiquity initrd=/casper/initrd.lz quiet splash --- maybe-ubiquity
LiveMediaBuild: Kubuntu 18.04 LTS "Bionic Beaver" - Release amd64 (20180426)
SourcePackage: ubiquity
UpgradeStatus: No upgrade log present (probably fresh install)