Installer Crash on format checkbox

OS: Kubuntu 18.10 amd64 Live USB
ubiquity version: 18.10.12

Steps to reproduce:
    1. Open the Kubuntu 18.10 installer.
    2. Get to the manual partitioning screen.
    3. Select an already existing ext4 partition.
    4. Click "Change..."
    5. From the "Use as" drop-down list, select "Ext 4 journaling file system".
    6. Make sure that the "Format the partition:" checkbox is unchecked.
    7. Click "OK".
    8. Click "Format?" checkbox for the previously selected ext4 partition.

Expected behavior:
    The checkbox becomes checked.

Actual behavior:
    A popup window informs the user that the installer has crashed with the following error:

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 18.10
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
CurrentDesktop: KDE
Date: Sat Oct 20 01:34:57 2018
InstallCmdLine: BOOT_IMAGE=/casper/vmlinuz file=/cdrom/preseed/kubuntu.seed boot=casper maybe-ubiquity quiet splash ---
LiveMediaBuild: Kubuntu 18.10 "Cosmic Cuttlefish" - Release amd64 (20181017.2)
SourcePackage: ubiquity
UpgradeStatus: No upgrade log present (probably fresh install)