Bug#990427: apt-listchanges: Invalid format text for mutt

[Brian Thompson <brian@hashvault.io>]

Package: apt-listchanges
Version: 3.24
Severity: normal

Dear Maintainer,

After performing an installation of a previously uninstalled package,
apt-listchanges reported an error:

"Invalid format text for mutt"

The package still installed, but no changelog or NEWS was shown or
emailed out.

The expected behavior is that no error is thrown since the email_format
is set correctly, or if there is an error in my configuration (aside
from the email_format field), then that be pointed out instead.

-- Package-specific info:
==> /etc/apt/listchanges.conf <==
[apt]
frontend=pager
which=both
email_address=brian@hashvault.io
email_format=text
confirm=false
headers=true
reverse=false
titled=true
save_seen=/var/lib/apt/listchanges.db
no_network=false

[cmdline]
frontend=pager
email_address=brian@hashvault.io
email_format=text
which=both

[custom]
frontend=browser
browser=chromium


-- System Information:
Debian Release: 11.0
  APT prefers testing-security
  APT policy: (500, 'testing-security'), (500, 'testing')
Architecture: amd64 (x86_64)

Kernel: Linux 5.10.0-7-amd64 (SMP w/8 CPU threads)
Locale: LANG=en_US.UTF-8, LC_CTYPE=en_US.UTF-8 (charmap=UTF-8), LANGUAGE not set
Shell: /bin/sh linked to /usr/bin/dash
Init: systemd (via /run/systemd/system)
LSM: AppArmor: enabled

Versions of packages apt-listchanges depends on:
ii  apt                    2.2.4
ii  debconf [debconf-2.0]  1.5.75
ii  python3                3.9.2-3
ii  python3-apt            2.2.0
ii  python3-debconf        1.5.75
ii  sensible-utils         0.0.14
ii  ucf                    3.0043

apt-listchanges recommends no packages.

Versions of packages apt-listchanges suggests:
ii  chromium [www-browser]                90.0.4430.212-1
ii  edbrowse [www-browser]                3.7.7-2
ii  firefox-esr [www-browser]             78.11.0esr-1
ii  kitty [x-terminal-emulator]           0.19.3-1
ii  links [www-browser]                   2.21-1+b1
ii  lynx [www-browser]                    2.9.0dev.6-2
ii  mlterm [x-terminal-emulator]          3.9.0-1
ii  palemoon [www-browser]                29.2.0-1.gtk2
ii  postfix [mail-transport-agent]        3.5.6-1+b1
ii  python3-gi                            3.38.0-2
ii  xfce4-terminal [x-terminal-emulator]  0.8.10-1
ii  xiterm+thai [x-terminal-emulator]     1.10-2+b2
ii  xterm [x-terminal-emulator]           366-1

-- debconf information:
* apt-listchanges/email-format: text
* apt-listchanges/confirm: false
* apt-listchanges/save-seen: true
* apt-listchanges/reverse: false
* apt-listchanges/email-address: brian@hashvault.io
* apt-listchanges/no-network: false
* apt-listchanges/which: both
* apt-listchanges/headers: true
* apt-listchanges/frontend: pager