Installer crashes when selecting format checkbox

[Tilman]

In the manual disk setup option I deleted the old Linux partition and then clicked it to create a new one. After that I selected the format checkbox for the partition which crashed the Installer with this message:

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
ApportVersion: 2.20.9-0ubuntu7
Architecture: amd64
CasperVersion: 1.394
Date: Thu May 24 15:44:34 2018
InstallCmdLine: BOOT_IMAGE=/casper/vmlinuz file=/cdrom/preseed/kubuntu.seed boot=casper maybe-ubiquity quiet splash ---
LiveMediaBuild: Kubuntu 18.04 LTS "Bionic Beaver" - Release amd64 (20180426)
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)