TypeError: expected string or bytes-like object in apport hook

[Marius Gedminas]

I see this on my machine when I try to report a Firefox bug:

$ ubuntu-bug firefox
ERROR: hook /usr/share/apport/package-hooks/source_firefox.py crashed:
Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/apport/report.py", line 205, in _run_hook
    symb['add_info'](report, ui)
  File "/usr/share/apport/package-hooks/source_firefox.py", line 1386, in add_info
    if (profile.current or profile.default) and profile.addon_compat_check_disabled:
  File "/usr/share/apport/package-hooks/source_firefox.py", line 1227, in addon_compat_check_disabled
    is_nightly = re.sub(r'^[^\.]+\.[0-9]+([a-z0-9]*).*', r'\1', self.last_version) == 'a1'
  File "/usr/lib/python3.7/re.py", line 192, in sub
    return _compile(pattern, flags).sub(repl, string, count)
TypeError: expected string or bytes-like object

ProblemType: Bug
DistroRelease: Ubuntu 19.10
Package: firefox 71.0+build5-0ubuntu0.19.10.1
ProcVersionSignature: Ubuntu 5.3.0-24.26-generic 5.3.10
Uname: Linux 5.3.0-24-generic x86_64
NonfreeKernelModules: zfs zunicode zavl icp zcommon znvpair
ApportVersion: 2.20.11-0ubuntu8.2
Architecture: amd64
BuildID: 20191205203726
CurrentDesktop: ubuntu:GNOME
Date: Wed Dec 11 09:26:03 2019
DefaultProfileExtensions: extensions.sqlite corrupt or missing
DefaultProfileIncompatibleExtensions: Unavailable (corrupt or non-existant compatibility.ini or extensions.sqlite)
DefaultProfileLocales: extensions.sqlite corrupt or missing
DefaultProfilePrefErrors: Unexpected character ',' before close parenthesis @ /usr/lib/firefox/omni.ja:greprefs.js:722
DefaultProfileThemes: extensions.sqlite corrupt or missing
EcryptfsInUse: Yes
InstallationDate: Installed on 2019-06-12 (181 days ago)
InstallationMedia: Ubuntu 19.04 "Disco Dingo" - Release amd64 (20190416)
Profiles:
 Profile1 (Default) - LastVersion=None/None (Out of date)
 Profile0 - LastVersion=71.0/20191205203726 (In use)
SourcePackage: firefox
UpgradeStatus: Upgraded to eoan on 2019-10-18 (53 days ago)