Manually partitioning a volume results in a crash

I am performing an install of Kubuntu 18.04 on an EEEPC 701 with 2GB RAM. The SSD isn't large enough to perform the install (4GB), so I have installed an external SD Card. On the partitioning page, I selected manual partitioning, and then configured the following:

````
/dev/sda1 - 1024MB /boot ext4 (Format auto-selected)
/dev/sda2 - 2974MB swap
/dev/sdb1 - 31913MB / ext4 (Format not selected)
````

When ticking the Format box, I got the following result:

````
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'
````

At this point, the installer halts.

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: ubiquity 18.04.14.6
ProcVersionSignature: Ubuntu 4.15.0-29.31-generic 4.15.18
Uname: Linux 4.15.0-29-generic i686
ApportVersion: 2.20.9-0ubuntu7.2
Architecture: i386
CasperVersion: 1.394
CurrentDesktop: KDE
Date: Thu Sep 6 10:23:53 2018
InstallCmdLine: file=/cdrom/preseed/kubuntu.seed boot=casper maybe-ubiquity initrd=/casper/initrd quiet splash --- maybe-ubiquity
LiveMediaBuild: Kubuntu 18.04.1 LTS "Bionic Beaver" - Release i386 (20180725)
SourcePackage: ubiquity
UpgradeStatus: No upgrade log present (probably fresh install)