Note: I am running MATE, but ukui volume control applet starts without
any warning or issue, but clicking leads to a crash.

If this is not supported, a proper warning on startup should be displayed
instead, or it should not start at all, or start required dependencies
automatically.

It looks like ukui-media implicitly depends on some stuff from
ukui-panel?

It should either be codified in package dependencies, or this dependency
in code removed. (Not sure why it need to do anything with settings or
panel, when doing a click on a apple icon, but maybe there is - i.e. to
show so kind of slider).

(gdb) r
Starting program: /usr/bin/ukui-volume-control-applet
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
[New Thread 0x7ffff5a006c0 (LWP 3200167)]
[New Thread 0x7ffff50006c0 (LWP 3200168)]
[New Thread 0x7fffefe006c0 (LWP 3200169)]
[New Thread 0x7fffef4006c0 (LWP 3200170)]
[Thread 0x7fffef4006c0 (LWP 3200170) exited]
[New Thread 0x7fffef4006c0 (LWP 3200171)]
[New Thread 0x7fffee8006c0 (LWP 3200172)]
[Thread 0x7fffef4006c0 (LWP 3200171) exited]
[New Thread 0x7fffef4006c0 (LWP 3200173)]
[New Thread 0x7fffede006c0 (LWP 3200174)]
[Thread 0x7fffee8006c0 (LWP 3200172) exited]
[Thread 0x7fffef4006c0 (LWP 3200173) exited]
[Thread 0x7fffede006c0 (LWP 3200174) exited]

(ukui-volume-control-applet:3200159): GLib-GIO-ERROR **: 14:08:05.134: Settings schema 'org.ukui.panel.toplevel' is not installed

Thread 1 "ukui-volume-con" received signal SIGTRAP, Trace/breakpoint trap.
g_log_structured_array (log_level=<optimized out>, fields=0x7fffffffcc90, n_fields=4) at ../../../glib/gmessages.c:563
563	../../../glib/gmessages.c: No such file or directory.
(gdb) btr
Undefined command: "btr".  Try "help".
(gdb) bt
#0  g_log_structured_array (log_level=<optimized out>, fields=0x7fffffffcc90, n_fields=4) at ../../../glib/gmessages.c:563
#1  0x00007ffff71f2bfe in g_log_default_handler
    (log_domain=log_domain@entry=0x7ffff7494b7c "GLib-GIO", log_level=log_level@entry=6, message=message@entry=0x555555aa6300 "Settings schema 'org.ukui.panel.toplevel' is not installed", unused_data=unused_data@entry=0x0) at ../../../glib/gmessages.c:3312
#2  0x00007ffff71f2e67 in g_logv (log_domain=0x7ffff7494b7c "GLib-GIO", log_level=G_LOG_LEVEL_ERROR, format=<optimized out>, args=<optimized out>) at ../../../glib/gmessages.c:1402
#3  0x00007ffff71f30ff in g_log
    (log_domain=log_domain@entry=0x7ffff7494b7c "GLib-GIO", log_level=log_level@entry=G_LOG_LEVEL_ERROR, format=format@entry=0x7ffff74b8c48 "Settings schema '%s' is not installed")
    at ../../../glib/gmessages.c:1471
#4  0x00007ffff743e85f in g_settings_set_property (object=0x555555aab060 [GSettings], prop_id=<optimized out>, value=<optimized out>, pspec=<optimized out>) at ../../../gio/gsettings.c:596
#5  0x00007ffff72ea53d in object_set_property
    (object=object@entry=0x555555aab060 [GSettings], pspec=0x55555598e3f0 [GParamString], value=0x7fffffffd1d0, nqueue=nqueue@entry=0x0, user_specified=<optimized out>)
    at ../../../gobject/gobject.c:1794
#6  0x00007ffff72eade8 in g_object_new_internal (class=class@entry=0x555555676280, params=params@entry=0x7fffffffd0d0, n_params=n_params@entry=2) at ../../../gobject/gobject.c:2273
#7  0x00007ffff72ec983 in g_object_new_valist (object_type=<optimized out>, first_property_name=<optimized out>, var_args=var_args@entry=0x7fffffffd3a0) at ../../../gobject/gobject.c:2567
#8  0x00007ffff72ecfd9 in g_object_new (object_type=<optimized out>, first_property_name=<optimized out>) at ../../../gobject/gobject.c:2040
#9  0x000055555555adcb in  ()
#10 0x00007ffff72e55a9 in _g_closure_invoke_va
    (closure=closure@entry=0x555555a68890, return_value=return_value@entry=0x0, instance=instance@entry=0x5555555c04f0, args=args@entry=0x7fffffffd6f0, n_params=0, param_types=0x0)
    at ../../../gobject/gclosure.c:895
#11 0x00007ffff72febbf in g_signal_emit_valist (instance=0x5555555c04f0, signal_id=192, detail=<optimized out>, var_args=var_args@entry=0x7fffffffd6f0) at ../../../gobject/gsignal.c:3456
#12 0x00007ffff72fedbf in g_signal_emit (instance=<optimized out>, signal_id=<optimized out>, detail=<optimized out>) at ../../../gobject/gsignal.c:3606
#13 0x00007ffff7a0b389 in  () at /lib/x86_64-linux-gnu/libgtk-3.so.0
#14 0x00007ffff76c2bb7 in  () at /lib/x86_64-linux-gnu/libgtk-3.so.0
#18 0x00007ffff72fedbf in <emit signal ??? on instance 0x5555559f6850 [GtkTrayIcon]> (instance=<optimized out>, signal_id=<optimized out>, detail=<optimized out>)
    at ../../../gobject/gsignal.c:3606
    #15 0x00007ffff72e53b0 in g_closure_invoke
    (closure=0x555555a67b30, return_value=return_value@entry=0x7fffffffd960, n_param_values=2, param_values=param_values@entry=0x7fffffffd9c0, invocation_hint=invocation_hint@entry=0x7fffffffd940) at ../../../gobject/gclosure.c:832
    #16 0x00007ffff72f8076 in signal_emit_unlocked_R
    (node=<optimized out>, detail=detail@entry=0, instance=instance@entry=0x5555559f6850, emission_return=emission_return@entry=0x7fffffffdab0, instance_and_params=instance_and_params@entry=0x7fffffffd9c0) at ../../../gobject/gsignal.c:3796
    #17 0x00007ffff72fe42d in g_signal_emit_valist (instance=<optimized out>, signal_id=<optimized out>, detail=<optimized out>, var_args=var_args@entry=0x7fffffffdb60)
    at ../../../gobject/gsignal.c:3559
#19 0x00007ffff799b4f4 in  () at /lib/x86_64-linux-gnu/libgtk-3.so.0
#20 0x00007ffff78397ee in  () at /lib/x86_64-linux-gnu/libgtk-3.so.0
#21 0x00007ffff783b2a6 in gtk_main_do_event () at /lib/x86_64-linux-gnu/libgtk-3.so.0
#22 0x00007ffff7565815 in  () at /lib/x86_64-linux-gnu/libgdk-3.so.0
#23 0x00007ffff75bf402 in  () at /lib/x86_64-linux-gnu/libgdk-3.so.0
#24 0x00007ffff71eb7a9 in g_main_dispatch (context=0x5555555c1a70) at ../../../glib/gmain.c:3454
#25 g_main_context_dispatch (context=context@entry=0x5555555c1a70) at ../../../glib/gmain.c:4172
#26 0x00007ffff71eba38 in g_main_context_iterate (context=0x5555555c1a70, block=block@entry=1, dispatch=dispatch@entry=1, self=<optimized out>) at ../../../glib/gmain.c:4248
#27 0x00007ffff71ebcef in g_main_loop_run (loop=0x555555a73690) at ../../../glib/gmain.c:4448
#28 0x00007ffff783a445 in gtk_main () at /lib/x86_64-linux-gnu/libgtk-3.so.0
#29 0x0000555555559f63 in main ()
(gdb) q
A debugging session is active.