Installation of xubuntu 16.04 from live cd hangs if /boot partition on other device with ext2 marked to format

I tried to install xubuntu 16.04. It took me about 2 hours!!! because of multiple errors I found in the GUI installation process. Because I wanted to install on already partitioned drive I choose advance path.

Reproduce
1) Run xubuntu live cd,
2) When OS will load all GUI push install xubuntu icon
3) Select Advance method
4) Assign partition /boot to external flashdisc with ext2 partition (I'm sure it's unmounted)
5) Now If you will mark to format that /boot partition installation process will not be able to format that drive without any message or warning. You can't go back, cancel the process so the only solution is one to reset computer or xkill GUI installation app. If you however uncheck 'formatting' /boot partition then installation process will go further without any problems.

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: installation-report (not installed)
ProcVersionSignature: Ubuntu 4.4.0-22.40-generic 4.4.8
Uname: Linux 4.4.0-22-generic x86_64
ApportVersion: 2.20.1-0ubuntu2.1
Architecture: amd64
CurrentDesktop: XFCE
Date: Fri Jun 10 18:43:54 2016
ProcEnviron:
 LANGUAGE=pl
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=pl_PL.UTF-8
 SHELL=/bin/bash
SourcePackage: installation-report
UpgradeStatus: No upgrade log present (probably fresh install)