Kubuntu installer crash everytime I click format checkbox

My brief history:
I'm trying to move to Linux. From Windows I created this part of 40gb unassigned space (I attached an image) just to prevent the loss of my files, this is to install Ubuntu on it but because I like this interface KDE that's why I preferred Kubuntu.

Now the problem is..:
I'm trying to install Kubuntu into this unallocated partition but when I get to "Disk Setup" in the installer I select this partition and then I click on "Add...", I select primary partition and root (/) as mount point. But then when you click on format checkbox, the installer crashes.

I'm trying not to format the whole disk since I already have a partition (sda1) with my files there.

This is the bug that I am getting from the installer. A screenshot is attached.

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 20 19:25:07 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)