settings window crash upon adding firefox icon to app list

similar issue with xfce4-notifyd-config, which both seem to list icons from apps in the running session. both crash upon adding the firefox icon from a desktop file - can repro from xfce4-session master git repo:

(gdb) r
Starting program: /home/landry/src/xfce4/xfce4-session/settings/xfce4-session-settings 

(xfce4-session-settings:22189): Gtk-WARNING **: 08:30:43.909: Content added to the action area of a dialog using header bars

(xfce4-session-settings:22189): Gtk-WARNING **: 08:30:43.910: Content added to the action area of a dialog using header bars
DBG[session-editor.c:472] manager_client_registered(): new client at /org/xfce/SessionClients/24d5121e4_4cb4_49c4_b1d9_c9333dbd943c
DBG[session-editor.c:505] manager_client_registered(): name xfwm4
DBG[session-editor.c:514] manager_client_registered(): hint 2
DBG[session-editor.c:524] manager_client_registered(): pid 42361
DBG[session-editor.c:519] manager_client_registered(): priority 15
DBG[session-editor.c:539] manager_client_registered(): adding 'xfwm4', obj path /org/xfce/SessionClients/24d5121e4_4cb4_49c4_b1d9_c9333dbd943c
DBG[session-editor.c:472] manager_client_registered(): new client at /org/xfce/SessionClients/2676ee046_770a_46d0_89eb_fab24e24442f
DBG[session-editor.c:505] manager_client_registered(): name xfce4-panel
DBG[session-editor.c:514] manager_client_registered(): hint 2
DBG[session-editor.c:524] manager_client_registered(): pid 9677
DBG[session-editor.c:519] manager_client_registered(): priority 25
DBG[session-editor.c:539] manager_client_registered(): adding 'xfce4-panel', obj path /org/xfce/SessionClients/2676ee046_770a_46d0_89eb_fab24e24442f
DBG[session-editor.c:472] manager_client_registered(): new client at /org/xfce/SessionClients/246bdbb90_eb16_4cf1_89de_693ea6ea7219
DBG[session-editor.c:505] manager_client_registered(): name xfsettingsd
DBG[session-editor.c:514] manager_client_registered(): hint 2
DBG[session-editor.c:524] manager_client_registered(): pid 8234
DBG[session-editor.c:519] manager_client_registered(): priority 20
DBG[session-editor.c:529] manager_client_registered(): desktop_file /etc/xdg/autostart/xfsettingsd.desktop
DBG[session-editor.c:539] manager_client_registered(): adding 'xfsettingsd', obj path /org/xfce/SessionClients/246bdbb90_eb16_4cf1_89de_693ea6ea7219
DBG[session-editor.c:472] manager_client_registered(): new client at /org/xfce/SessionClients/221da1259_d93f_40e3_aaa9_469555335ad2
DBG[session-editor.c:505] manager_client_registered(): name xfdesktop
DBG[session-editor.c:514] manager_client_registered(): hint 2
DBG[session-editor.c:524] manager_client_registered(): pid 9265
DBG[session-editor.c:519] manager_client_registered(): priority 35
DBG[session-editor.c:539] manager_client_registered(): adding 'xfdesktop', obj path /org/xfce/SessionClients/221da1259_d93f_40e3_aaa9_469555335ad2
DBG[session-editor.c:472] manager_client_registered(): new client at /org/xfce/SessionClients/2a02d9b3b_c174_40b5_a36d_1292da4de5c0
DBG[session-editor.c:505] manager_client_registered(): name /usr/local/bin/nextcloud
DBG[session-editor.c:514] manager_client_registered(): hint 0
DBG[session-editor.c:519] manager_client_registered(): priority 50
DBG[session-editor.c:539] manager_client_registered(): adding '/usr/local/bin/nextcloud', obj path /org/xfce/SessionClients/2a02d9b3b_c174_40b5_a36d_1292da4de5c0
DBG[session-editor.c:472] manager_client_registered(): new client at /org/xfce/SessionClients/21d8ddb13_42c2_4e60_afad_fe52250da31b
DBG[session-editor.c:505] manager_client_registered(): name keepassxc
DBG[session-editor.c:514] manager_client_registered(): hint 0
DBG[session-editor.c:519] manager_client_registered(): priority 50
DBG[session-editor.c:539] manager_client_registered(): adding 'keepassxc', obj path /org/xfce/SessionClients/21d8ddb13_42c2_4e60_afad_fe52250da31b
DBG[session-editor.c:472] manager_client_registered(): new client at /org/xfce/SessionClients/2340c2115_e105_4961_959d_afee65b11bb2
DBG[session-editor.c:505] manager_client_registered(): name firefox
DBG[session-editor.c:514] manager_client_registered(): hint 0
DBG[session-editor.c:519] manager_client_registered(): priority 50
DBG[session-editor.c:539] manager_client_registered(): adding 'firefox', obj path /org/xfce/SessionClients/2340c2115_e105_4961_959d_afee65b11bb2
[New thread 405619]
[New thread 144455]
[New thread 497902]

Thread 1 received signal SIGSEGV, Segmentation fault.
strlen () at /usr/src/lib/libc/arch/amd64/string/strlen.S:125
125	2:
(gdb) bt
#0  strlen () at /usr/src/lib/libc/arch/amd64/string/strlen.S:125
#1  0x0000064fdb83d234 in g_strdup (str=0x64f915ba19a '\337' <repeats 199 times>, <incomplete sequence \337>...) at ../glib-2.66.2/glib/gstrfuncs.c:363
#2  0x0000064ffe04d88b in value_collect_string (value=0x7f7ffffd3270, n_collect_values=65535, collect_values=0x0, collect_flags=538976288) at ../glib-2.66.2/gobject/gvaluetypes.c:293
#3  0x0000064ffe02a145 in g_object_new_valist (object_type=<optimized out>, first_property_name=<optimized out>, var_args=0x7f7ffffd34e0) at ../glib-2.66.2/gobject/gobject.c:2250
#4  0x0000064ffe029c2a in g_object_new (object_type=6938986622720, first_property_name=0xffff <error: Cannot access memory at address 0xffff>) at ../glib-2.66.2/gobject/gobject.c:1782
#5  0x0000064f889a0d83 in g_themed_icon_new_with_default_fallbacks (iconname=0x64f915ba19a '\337' <repeats 199 times>, <incomplete sequence \337>...) at ../glib-2.66.2/gio/gthemedicon.c:444
#6  0x0000064ff7d4186d in IA__xfce_gicon_from_name (name=0x64f7d19bf20 "firefox") at xfce-gtk-extensions.c:885
#7  0x0000064d720cd9fe in manager_client_registered (proxy=<optimized out>, object_path=0x650059d7780 "/org/xfce/SessionClients/2340c2115_e105_4961_959d_afee65b11bb2", user_data=0x64fc70a63e0)
    at session-editor.c:557
#8  0x0000064d720cdeef in session_editor_populate_treeview (treeview=<optimized out>) at session-editor.c:868
#9  session_editor_init (builder=0x65046b226b0) at session-editor.c:885
#10 0x0000064d720ccb1e in main (argc=1, argv=0x7f7ffffd38f8) at main.c:159

the firefox.desktop file i have has a 'valid' icon:

[08:33] nikki:~/ $grep -i icon /usr/local/share/applications/firefox.desktop
Icon=/usr/local/lib/firefox/browser/chrome/icons/default/default128.png
[08:34] nikki:~/ $ls -l /usr/local/lib/firefox/browser/chrome/icons/default/default128.png
-rw-r--r--  1 root  bin  13513 Nov 10 18:07 /usr/local/lib/firefox/browser/chrome/icons/default/default128.png

my understanding of libxfce4ui codepaths is that xfce_icon_name_from_desktop_id fails with this icon.