In-place upgrading from Pop!_OS to Kubuntu had a weird error

[Hamza Lakhani]

While in-place upgrading from Pop!_OS to Kubuntu, there was an error with re-installing my apps. And then the installer threw an error and asked me to debug it accordingly.

ProblemType: Bug
DistroRelease: Ubuntu 20.04
Package: ubiquity 20.04.15.10 [modified: lib/partman/automatically_partition/question]
ProcVersionSignature: Ubuntu 5.8.0-43.49~20.04.1-generic 5.8.18
Uname: Linux 5.8.0-43-generic x86_64
ApportVersion: 2.20.11-0ubuntu27.16
Architecture: amd64
CasperMD5CheckResult: skip
CasperVersion: 1.445.1
CurrentDesktop: KDE
Date: Thu Jul 15 18:25:24 2021
InstallCmdLine: persistent file=/cdrom/preseed/kubuntu.seed maybe-ubiquity initrd=/casper/initrd quiet splash --- maybe-ubiquity
LiveMediaBuild: Kubuntu 20.04.2.0 LTS "Focal Fossa" - Release amd64 (20210209.1)
SourcePackage: ubiquity
UpgradeStatus: No upgrade log present (probably fresh install)

[Ubuntu Foundations Team Bug Bot (crichton)]

Exception during installation:
Jul 15 18:24:55 kubuntu /plugininstall.py: Traceback (most recent call last):
Jul 15 18:24:55 kubuntu /plugininstall.py: File "/usr/share/ubiquity/plugininstall.py", line 252, in run
Jul 15 18:24:55 kubuntu /plugininstall.py: self.apt_clone_restore()
Jul 15 18:24:55 kubuntu /plugininstall.py: File "/usr/share/ubiquity/plugininstall.py", line 1523, in apt_clone_restore
Jul 15 18:24:55 kubuntu /plugininstall.py: subprocess.check_call(
Jul 15 18:24:55 kubuntu /plugininstall.py: File "/usr/lib/python3.8/subprocess.py", line 364, in check_call
Jul 15 18:24:55 kubuntu /plugininstall.py: raise CalledProcessError(retcode, cmd)
Jul 15 18:24:55 kubuntu /plugininstall.py: subprocess.CalledProcessError: Command '['apt-clone', 'restore-new-distro', '/target/ubiquity-apt-clone/apt-clone-state-kubuntu.tar.gz', 'focal', '--destination', '/target']' returned non-zero exit status 1.
Jul 15 18:24:55 kubuntu /plugininstall.py: