Ubiquity crashes on while installing Ubuntu 20.04

[PJSingh5000]

Ubiquity crashes on while installing Ubuntu 20.04

Options selected:
~ Normal install
~ Do not download updates
~ Something else
~ Selected install partition, efi partition, and mount points for two additional partitions
~ All partitions were ext4 (except efi partition)

Ubiquity crashed when the install was almost complete.

Ubiquity displayed "Configuring apt" expander.

The following output was visible on the terminal window below the expander:

May 23 02:08:10 ubuntu /plugininstall.py: ubiquity.install_misc.InstallStepError: AptSetup failed with code 1

May 23 02:08:10 ubuntu /plugininstall.py

/usr/lib/ubiquity/ubiquity/frontend/gtk_components/nmwidgets.py:17: Warning: Source ID 8302 was not found when attempting to remove it
  GLib.source_remove(self.timeout_id)

/usr/lib/ubiquity/ubiquity/frontend/gtk_components/nmwidgets.py:133: Warning: Source ID 8475 was not found when attempting to remove it
  GLib.source_remove(self.rows_changed_id)

ProblemType: Bug
DistroRelease: Ubuntu 20.04
Package: ubiquity 20.04.15
ProcVersionSignature: Ubuntu 5.4.0-26.30-generic 5.4.30
Uname: Linux 5.4.0-26-generic x86_64
NonfreeKernelModules: zfs zunicode zavl icp zcommon znvpair
ApportVersion: 2.20.11-0ubuntu27
Architecture: amd64
CasperMD5CheckResult: pass
CasperVersion: 1.445
Date: Sat May 23 02:09:41 2020
InstallCmdLine: BOOT_IMAGE=(loop)/casper/vmlinuz boot=casper iso-scan/filename=/ubuntu.iso noprompt noeject
LiveMediaBuild: Ubuntu 20.04.0 2020.05.23 LTS "Custom Focal Fossa" (20200523)
SourcePackage: ubiquity
UpgradeStatus: No upgrade log present (probably fresh install)

[Cubic PPA (cubic-wizard)]

The issue was because the customized filesystem.squashfs was missing the /proc and /tmp directories.
Ubiquity expects these directories to be present, because they are not automatically created during the installation process.
Including empty /proc and /tmp (and /run) directories solved the problem.