Core dumps when adding or changing regexes

[Captain Chaos]

When adding or editing a regex (in colour mode), multitail consistently core dumps.

1. Start multitail as follows: multitail worldpainter-access.log
2. Press e
3. Press a
4. Enter 2\.10\.13
5. Press c for colour usage
6. Press y to negate
7. Press q to close the regexes window

At this point the program consistently core dumps for me.

lsb_release -a:
Distributor ID: Ubuntu
Description: Ubuntu 20.04.4 LTS
Release: 20.04
Codename: focal

uname -a:
Linux server07.pepsoft.org 5.4.0-121-generic #137-Ubuntu SMP Wed Jun 15 13:33:07 UTC 2022 x86_64 x86_64 x86_64 GNU/Linux

ProblemType: Bug
DistroRelease: Ubuntu 20.04
Package: multitail 6.4.2-3build2
ProcVersionSignature: Ubuntu 5.4.0-121.137-generic 5.4.189
Uname: Linux 5.4.0-121-generic x86_64
ApportVersion: 2.20.11-0ubuntu27.24
Architecture: amd64
CasperMD5CheckResult: skip
Date: Sat Jul 16 16:12:56 2022
InstallationDate: Installed on 2016-03-30 (2299 days ago)
InstallationMedia: Ubuntu-Server 16.04 LTS "Xenial Xerus" - Beta amd64 (20160325)
ProcEnviron:
 LANGUAGE=en_US:en
 TERM=xterm
 PATH=(custom, no user)
 LANG=en_US.UTF-8
 SHELL=/bin/bash
SourcePackage: multitail
UpgradeStatus: Upgraded to focal on 2021-12-04 (224 days ago)

[Captain Chaos (launchpad-chaos)]

Even though it says it core dumps, I don't see a core file in the current directory.

[Captain Chaos (launchpad-chaos)]

If I press n to not negate, it does not core dump, so it seems to be related. This is while connected via PuTTY. TERM is set to xterm and LANG to en_US.UTF-8.