$ G_DEBUG=fatal-criticals gdb xfce4-appearance-settings 
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This GDB was configured as "i386-marcel-freebsd"...
(gdb) run
Starting program: /usr/local/bin/xfce4-appearance-settings 
[New LWP 100107]
[New Thread 29d01140 (LWP 100107)]

GLib-CRITICAL **: g_convert: assertion `str != NULL' failed
aborting...

Program received signal SIGABRT, Aborted.
[Switching to Thread 29d01140 (LWP 100107)]
0x286ecf47 in kill () from /lib/libc.so.7
(gdb) bt full
#0  0x286ecf47 in kill () from /lib/libc.so.7
No symbol table info available.
#1  0x285f9347 in raise () from /lib/libthr.so.3
No symbol table info available.
#2  0x286ebaaa in abort () from /lib/libc.so.7
No symbol table info available.
#3  0x2857eb25 in g_logv () from /usr/local/lib/libglib-2.0.so.0
No symbol table info available.
#4  0x2857eb69 in g_log () from /usr/local/lib/libglib-2.0.so.0
No symbol table info available.
#5  0x2857edab in g_return_if_fail_warning () from /usr/local/lib/libglib-2.0.so.0
No symbol table info available.
#6  0x28560204 in g_convert () from /usr/local/lib/libglib-2.0.so.0
No symbol table info available.
#7  0x285a55a9 in g_utf8_collate () from /usr/local/lib/libglib-2.0.so.0
No symbol table info available.
#8  0x282b4c25 in gtk_tooltip_trigger_tooltip_query () from /usr/local/lib/libgtk-x11-2.0.so.0
No symbol table info available.
#9  0x281d7567 in gtk_link_button_new_with_label () from /usr/local/lib/libgtk-x11-2.0.so.0
No symbol table info available.
#10 0x281d79b6 in gtk_link_button_new_with_label () from /usr/local/lib/libgtk-x11-2.0.so.0
No symbol table info available.
#11 0x281d972c in gtk_list_store_set_valist () from /usr/local/lib/libgtk-x11-2.0.so.0
No symbol table info available.
#12 0x281d9792 in gtk_list_store_set () from /usr/local/lib/libgtk-x11-2.0.so.0
No symbol table info available.
#13 0x0804a7a7 in appearance_settings_load_ui_themes (list_store=0x29d4f558, tree_view=0x29dc90e8) at main.c:459
	dir = (GDir *) 0x29d0a308
	tree_path = Variable "tree_path" is not available.
(gdb)