I don't go into gufw very often so I'm not sure when this started, but trying to run it today I noticed it failed to appear. Looking in journalctl I see that it made python3 coredump:
May 17 00:50:36 hostname python3[329779]: gdk_keymap_get_for_display: assertion 'GDK_IS_DISPLAY (display)' failed
May 17 00:50:36 hostname python3[329779]: gdk_keymap_get_modifier_mask: assertion 'GDK_IS_KEYMAP (keymap)' failed
May 17 00:50:36 hostname python3[329779]: gdk_keymap_get_for_display: assertion 'GDK_IS_DISPLAY (display)' failed
May 17 00:50:36 hostname python3[329779]: _gtk_replace_virtual_modifiers: assertion 'GDK_IS_KEYMAP (keymap)' failed
May 17 00:50:36 hostname python3[329779]: gdk_keymap_get_for_display: assertion 'GDK_IS_DISPLAY (display)' failed
May 17 00:50:36 hostname python3[329779]: gdk_keymap_get_modifier_mask: assertion 'GDK_IS_KEYMAP (keymap)' failed
May 17 00:50:36 hostname python3[329779]: gdk_keymap_get_for_display: assertion 'GDK_IS_DISPLAY (display)' failed
May 17 00:50:36 hostname python3[329779]: _gtk_replace_virtual_modifiers: assertion 'GDK_IS_KEYMAP (keymap)' failed
May 17 00:50:36 hostname python3[329779]: gdk_keymap_get_for_display: assertion 'GDK_IS_DISPLAY (display)' failed
May 17 00:50:36 hostname python3[329779]: gdk_keymap_get_modifier_mask: assertion 'GDK_IS_KEYMAP (keymap)' failed
May 17 00:50:36 hostname python3[329779]: gdk_keymap_get_for_display: assertion 'GDK_IS_DISPLAY (display)' failed
May 17 00:50:36 hostname python3[329779]: _gtk_replace_virtual_modifiers: assertion 'GDK_IS_KEYMAP (keymap)' failed
May 17 00:50:36 hostname python3[329779]: gdk_keymap_get_for_display: assertion 'GDK_IS_DISPLAY (display)' failed
May 17 00:50:36 hostname python3[329779]: gdk_keymap_get_modifier_mask: assertion 'GDK_IS_KEYMAP (keymap)' failed
May 17 00:50:36 hostname python3[329779]: gdk_keymap_get_for_display: assertion 'GDK_IS_DISPLAY (display)' failed
May 17 00:50:36 hostname python3[329779]: _gtk_replace_virtual_modifiers: assertion 'GDK_IS_KEYMAP (keymap)' failed
May 17 00:50:36 hostname python3[329779]: gdk_keymap_get_for_display: assertion 'GDK_IS_DISPLAY (display)' failed
May 17 00:50:36 hostname python3[329779]: gdk_keymap_get_modifier_mask: assertion 'GDK_IS_KEYMAP (keymap)' failed
May 17 00:50:36 hostname python3[329779]: gdk_keymap_get_for_display: assertion 'GDK_IS_DISPLAY (display)' failed
May 17 00:50:36 hostname python3[329779]: _gtk_replace_virtual_modifiers: assertion 'GDK_IS_KEYMAP (keymap)' failed
May 17 00:50:36 hostname python3[329779]: gdk_keymap_get_for_display: assertion 'GDK_IS_DISPLAY (display)' failed
May 17 00:50:36 hostname python3[329779]: gdk_keymap_get_modifier_mask: assertion 'GDK_IS_KEYMAP (keymap)' failed
May 17 00:50:36 hostname python3[329779]: gdk_keymap_get_for_display: assertion 'GDK_IS_DISPLAY (display)' failed
May 17 00:50:36 hostname python3[329779]: _gtk_replace_virtual_modifiers: assertion 'GDK_IS_KEYMAP (keymap)' failed
May 17 00:50:36 hostname python3[329779]: gdk_keymap_get_for_display: assertion 'GDK_IS_DISPLAY (display)' failed
May 17 00:50:36 hostname python3[329779]: gdk_keymap_get_modifier_mask: assertion 'GDK_IS_KEYMAP (keymap)' failed
May 17 00:50:36 hostname python3[329779]: gdk_keymap_get_for_display: assertion 'GDK_IS_DISPLAY (display)' failed
May 17 00:50:36 hostname python3[329779]: _gtk_replace_virtual_modifiers: assertion 'GDK_IS_KEYMAP (keymap)' failed
May 17 00:50:36 hostname python3[329779]: gdk_keymap_get_for_display: assertion 'GDK_IS_DISPLAY (display)' failed
May 17 00:50:36 hostname python3[329779]: gdk_keymap_get_modifier_mask: assertion 'GDK_IS_KEYMAP (keymap)' failed
May 17 00:50:36 hostname python3[329779]: gdk_keymap_get_for_display: assertion 'GDK_IS_DISPLAY (display)' failed
May 17 00:50:36 hostname python3[329779]: _gtk_replace_virtual_modifiers: assertion 'GDK_IS_KEYMAP (keymap)' failed
May 17 00:50:36 hostname python3[329779]: gdk_keymap_get_for_display: assertion 'GDK_IS_DISPLAY (display)' failed
May 17 00:50:36 hostname python3[329779]: gdk_keymap_get_modifier_mask: assertion 'GDK_IS_KEYMAP (keymap)' failed
May 17 00:50:36 hostname python3[329779]: gdk_keymap_get_for_display: assertion 'GDK_IS_DISPLAY (display)' failed
May 17 00:50:36 hostname python3[329779]: _gtk_replace_virtual_modifiers: assertion 'GDK_IS_KEYMAP (keymap)' failed
May 17 00:50:36 hostname python3[329779]: gdk_keymap_get_for_display: assertion 'GDK_IS_DISPLAY (display)' failed
May 17 00:50:36 hostname python3[329779]: gdk_keymap_get_modifier_mask: assertion 'GDK_IS_KEYMAP (keymap)' failed
May 17 00:50:36 hostname python3[329779]: gdk_keymap_get_for_display: assertion 'GDK_IS_DISPLAY (display)' failed
May 17 00:50:36 hostname python3[329779]: _gtk_replace_virtual_modifiers: assertion 'GDK_IS_KEYMAP (keymap)' failed
May 17 00:50:36 hostname gufw.desktop[329779]: /usr/lib64/python3.11/site-packages/gufw/gufw/view/gufw.py:77: Warning: g_object_set_data_full: assertion 'G_IS_OBJECT (object)' failed
May 17 00:50:36 hostname gufw.desktop[329779]: self.builder.add_from_file('/usr/share/gufw/ui/gufw.ui')
May 17 00:50:36 hostname python3[329779]: _gtk_css_lookup_resolve: assertion '(((__extension__ ({ GTypeInstance *__inst = (GTypeInstance*) ((provider)); GType __t = ((_gtk_style_provider_private_get_type ())); gboolean __r; if (!__inst) __r = (0); else if (__inst->g_class && __inst->g_class->g_type == __t) __r = (!(0)); else __r = g_type_check_instance_is_a (__inst, __t); __r; }))))' failed
May 17 00:50:36 hostname kernel: traps: python3[329779] trap int3 ip:7f7c11f608db sp:7ffc21fd77a0 error:0 in libglib-2.0.so.0.8000.2[7f7c11f17000+9e000]
May 17 00:50:36 hostname kernel: audit: type=1701 audit(1715921436.944:531): auid=1000 uid=0 gid=0 ses=2 subj=unconfined pid=329779 comm="python3" exe="/usr/bin/python3.11" sig=5 res=1
May 17 00:50:36 hostname python3[329779]: Can't create a GtkStyleContext without a display connection
May 17 00:50:36 hostname systemd[1]: Created slice system-systemd\x2dcoredump.slice - Slice /system/systemd-coredump.
May 17 00:50:36 hostname kernel: audit: type=1334 audit(1715921436.951:532): prog-id=125 op=LOAD
May 17 00:50:36 hostname kernel: audit: type=1334 audit(1715921436.951:533): prog-id=126 op=LOAD
May 17 00:50:36 hostname kernel: audit: type=1334 audit(1715921436.951:534): prog-id=127 op=LOAD
May 17 00:50:36 hostname systemd[1]: Started systemd-coredump@0-329795-0.service - Process Core Dump (PID 329795/UID 0).
May 17 00:50:36 hostname kernel: audit: type=1130 audit(1715921436.965:535): pid=1 uid=0 auid=4294967295 ses=4294967295 subj=unconfined msg='unit=systemd-coredump@0-329795-0 comm="systemd" exe="/usr/lib64/systemd/systemd" hostname=? addr=? terminal=? res=success'
May 17 00:50:37 hostname systemd-coredump[329796]: Process 329779 (python3) of user 0 dumped core.
Stack trace of thread 329779:
#0 0x00007f7c11f608db g_log_writer_default (libglib-2.0.so.0 + 0x668db)
#1 0x00007f7c11f5da88 g_log_structured_array (libglib-2.0.so.0 + 0x63a88)
#2 0x00007f7c11f5e75b g_log_structured_standard (libglib-2.0.so.0 + 0x6475b)
#3 0x00007f7c10638758 n/a (libgtk-3.so.0 + 0x238758)
#4 0x00007f7c11e98c51 g_type_create_instance (libgobject-2.0.so.0 + 0x36c51)
#5 0x00007f7c11e7b645 n/a (libgobject-2.0.so.0 + 0x19645)
#6 0x00007f7c11e7d5b7 g_object_new_with_properties (libgobject-2.0.so.0 + 0x1b5b7)
#7 0x00007f7c11e7e51a g_object_new (libgobject-2.0.so.0 + 0x1c51a)
#8 0x00007f7c107207a7 n/a (libgtk-3.so.0 + 0x3207a7)
#9 0x00007f7c10720835 gtk_style_new (libgtk-3.so.0 + 0x320835)
#10 0x00007f7c10722e12 gtk_widget_get_default_style (libgtk-3.so.0 + 0x322e12)
#11 0x00007f7c106be63f n/a (libgtk-3.so.0 + 0x2be63f)
#12 0x00007f7c11e98c13 g_type_create_instance (libgobject-2.0.so.0 + 0x36c13)
#13 0x00007f7c11e7b645 n/a (libgobject-2.0.so.0 + 0x19645)
#14 0x00007f7c11e7d95c g_object_newv (libgobject-2.0.so.0 + 0x1b95c)
#15 0x00007f7c104a9913 n/a (libgtk-3.so.0 + 0xa9913)
#16 0x00007f7c104aa946 n/a (libgtk-3.so.0 + 0xaa946)
#17 0x00007f7c104ac385 n/a (libgtk-3.so.0 + 0xac385)
#18 0x00007f7c11f59647 n/a (libglib-2.0.so.0 + 0x5f647)
#19 0x00007f7c11f5a462 g_markup_parse_context_parse (libglib-2.0.so.0 + 0x60462)
#20 0x00007f7c104ac728 n/a (libgtk-3.so.0 + 0xac728)
#21 0x00007f7c104a71d6 gtk_builder_add_from_file (libgtk-3.so.0 + 0xa71d6)
#22 0x00007f7c11e5cc2d n/a (libffi.so.7 + 0x6c2d)
#23 0x00007f7c11e5c176 n/a (libffi.so.7 + 0x6176)
#24 0x00007f7c1208d080 n/a (_gi.cpython-311-x86_64-linux-gnu.so + 0x2c080)
#25 0x00007f7c1208ef5d n/a (_gi.cpython-311-x86_64-linux-gnu.so + 0x2df5d)
#26 0x00007f7c1207db9b n/a (_gi.cpython-311-x86_64-linux-gnu.so + 0x1cb9b)
#27 0x00007f7c1295745c _PyObject_MakeTpCall (libpython3.11.so.1.0 + 0x15745c)
#28 0x00007f7c12963cb3 _PyEval_EvalFrameDefault (libpython3.11.so.1.0 + 0x163cb3)
#29 0x00007f7c1298bd30 _PyFunction_Vectorcall (libpython3.11.so.1.0 + 0x18bd30)
#30 0x00007f7c129937ca n/a (libpython3.11.so.1.0 + 0x1937ca)
#31 0x00007f7c1295749b _PyObject_MakeTpCall (libpython3.11.so.1.0 + 0x15749b)
#32 0x00007f7c12963cb3 _PyEval_EvalFrameDefault (libpython3.11.so.1.0 + 0x163cb3)
#33 0x00007f7c12a274f4 n/a (libpython3.11.so.1.0 + 0x2274f4)
#34 0x00007f7c12a26e88 PyEval_EvalCode (libpython3.11.so.1.0 + 0x226e88)
#35 0x00007f7c12a454d3 n/a (libpython3.11.so.1.0 + 0x2454d3)
#36 0x00007f7c12a4120a n/a (libpython3.11.so.1.0 + 0x24120a)
#37 0x00007f7c12a6da23 n/a (libpython3.11.so.1.0 + 0x26da23)
#38 0x00007f7c12a6cb21 _PyRun_SimpleFileObject (libpython3.11.so.1.0 + 0x26cb21)
#39 0x00007f7c12a6c764 _PyRun_AnyFileObject (libpython3.11.so.1.0 + 0x26c764)
#40 0x00007f7c12a5123c Py_RunMain (libpython3.11.so.1.0 + 0x25123c)
#41 0x00007f7c12a17017 Py_BytesMain (libpython3.11.so.1.0 + 0x217017)
#42 0x00007f7c1264030e __libc_start_call_main (libc.so.6 + 0x2a30e)
#43 0x00007f7c126403c9 __libc_start_main_impl (libc.so.6 + 0x2a3c9)
#44 0x000055de34847065 _start (python3.11 + 0x1065)
ELF object binary architecture: AMD x86-64
May 17 00:50:37 hostname systemd[1]: systemd-coredump@0-329795-0.service: Deactivated successfully.
May 17 00:50:37 hostname kernel: audit: type=1131 audit(1715921437.077:536): pid=1 uid=0 auid=4294967295 ses=4294967295 subj=unconfined msg='unit=systemd-coredump@0-329795-0 comm="systemd" exe="/usr/lib64/systemd/systemd" hostname=? addr=? terminal=? res=success'
May 17 00:50:37 hostname gufw.desktop[329687]: /bin/gufw-pkexec: line 3: 329779 Trace/breakpoint trap (core dumped) python3 /usr/lib/python${PYTHON3_VERSION}/site-packages/gufw/gufw.py $1
May 17 00:50:37 hostname kernel: audit: type=1334 audit(1715921437.128:537): prog-id=127 op=UNLOAD
May 17 00:50:37 hostname kernel: audit: type=1334 audit(1715921437.128:538): prog-id=126 op=UNLOAD
May 17 00:50:37 hostname kernel: audit: type=1334 audit(1715921437.128:539): prog-id=125 op=UNLOAD
I wasn't sure whether to post this in the issues for the gufw package or one of the python packages so I posted it here, feel free to forward me to report it as an issue somewhere.