shares-admin crashed with SIGSEGV in g_type_check_instance_cast()

Steps to reproduce:
1. Have Ubuntu 18.04 LTS (or newer) installed
2. Install all necessary dependencies by

   sudo apt-get install samba nfs-kernel-server

3. Launch `shares-admin`, click Unlock, enter password, click Add

Expected results:
* something useful happens

Actual results:
* `shares-admin` crashed with segmentation fault.

ProblemType: Crash
DistroRelease: Ubuntu 21.04
Package: gnome-system-tools 3.0.0-9.1ubuntu1
ProcVersionSignature: Ubuntu 5.11.0-18.19-generic 5.11.17
Uname: Linux 5.11.0-18-generic x86_64
ApportVersion: 2.20.11-0ubuntu65.1
Architecture: amd64
CasperMD5CheckResult: pass
CurrentDesktop: MATE
Date: Sat Jul 31 11:27:54 2021
ExecutablePath: /usr/bin/shares-admin
ExecutableTimestamp: 1612536482
InstallationDate: Installed on 2021-04-23 (98 days ago)
InstallationMedia: Ubuntu-MATE 21.04 "Hirsute Hippo" - Release amd64 (20210420)
ProcCmdline: shares-admin
ProcCwd: /home/h
SegvAnalysis:
 Segfault happened at: 0x7fb9b536cb1c: movzbl 0x14(%rbx),%ecx
 PC (0x7fb9b536cb1c) ok
 source "0x14(%rbx)" (0xffffffff9c7d31b4) not located in a known VMA region (needed readable region)!
 destination "%ecx" ok
SegvReason: reading unknown VMA
Signal: 11
SourcePackage: gnome-system-tools
StacktraceTop:
 ?? () from /lib/x86_64-linux-gnu/libgobject-2.0.so.0
 g_type_check_instance_cast () from /lib/x86_64-linux-gnu/libgobject-2.0.so.0
 share_settings_dialog_run_for_iter ()
 share_settings_dialog_run ()
 g_signal_emit_valist () from /lib/x86_64-linux-gnu/libgobject-2.0.so.0
Title: shares-admin crashed with SIGSEGV in g_type_check_instance_cast()
UpgradeStatus: No upgrade log present (probably fresh install)
UserGroups: adm cdrom dip lpadmin lxd plugdev sambashare sudo
XsessionErrors:
 mate-session[1446]: WARNING: Unable to find provider '' of required component 'dock'
 (caja:1808): Gtk-WARNING **: 11:27:31.338: Failed to register client: GDBus.Error:org.gnome.SessionManager.AlreadyRegistered: Unable to register client
 (process:1967): ayatana-indicator-sound-WARNING **: 11:27:31.556: volume-control-pulse.vala:745: Unable to connect to dbus server at 'unix:path=/run/user/1000/pulse/dbus-socket': Could not connect: No such file or directory
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