[Boxes] The installer crashes when installing with `Express Installation ON` in `Boxes` when a Japanese Keyboard is connected.

[ichro furuya]

### situation:

I have a Japanese Keyboard connected to my PC. Launched the `Boxes` app. I chose `Ubuntu 20.10 x86_64(live)` and installed with `Express Installation: ON`.

I answered `Yes` to the question `Are you sure you want to keep the default keyboard layout (ja)?`. About the time the installation progressed and reached the `keyboard settings`, the installer crashed.

### usage environment:

Host OS: xubuntu 20.04.4 LTS(Japanese environment):

```
$ uname -r
5.15.0-41-generic
```

Boxes version: 42.3-17184268

```
$ flatpak list
:
Boxes org.gnome.Boxes 42.3 stable system
```

### If checked after installation:

```
$ hostnamectl
:
Architecture: x86-64
Hardware Vender: QEMU
Hardware Model: Standard PC_Q35 + ICH9,2009_

$ localectl
System Locale: LANG-ja_JP.UTF-8
VC Keymap: n/a
X11 Layout: jp
X11 Model: pc105
```

### Frequency of occurrence:

always

ProblemType: Bug
DistroRelease: Ubuntu 21.10
Package: ubiquity 21.10.10
ProcVersionSignature: Ubuntu 5.13.0-19.19-generic 5.13.14
Uname: Linux 5.13.0-19-generic x86_64
NonfreeKernelModules: zfs zunicode zavl icp zcommon znvpair
ApportVersion: 2.20.11-0ubuntu70
Architecture: amd64
CasperMD5CheckResult: pass
CasperVersion: 1.465
CurrentDesktop: ubuntu:GNOME
Date: Wed Aug 3 17:50:38 2022
InstallCmdLine: file=/preseed.cfg locale=ja keyboard-configuration/layoutcode=ja hostname=ubuntu2110-2 priority=critical console-setup/ask_detect=false domain=unassigned-domain interface=auto noprompt boot=casper automatic-ubiquity
LiveMediaBuild: Ubuntu 21.10 "Impish Indri" - Release amd64 (20211012)
ProcEnviron:
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=ja_JP.UTF-8
 LC_NUMERIC=C.UTF-8
SourcePackage: ubiquity
UpgradeStatus: No upgrade log present (probably fresh install)

[ichro furuya (furuya)]

Same thing happens on `Ubuntu 22.04 LTS x86_64 (live)`.