STEPS TO REPRODUCE 1. Navigate to System Settings > Application Style > GNOME/GTK Application style 2. Change the GTK2 or GTK3 theme from anything to anything 3. Click apply OBSERVED RESULT System Settings freezes indefinitely. The console log says the following: (systemsettings5:31664): GLib-GObject-WARNING **: 15:46:39.553: cannot register existing type 'GdkDisplayManager' (systemsettings5:31664): GLib-CRITICAL **: 15:46:39.553: g_once_init_leave: assertion 'result != 0' failed (systemsettings5:31664): GLib-GObject-CRITICAL **: 15:46:39.553: g_object_new_with_properties: assertion 'G_TYPE_IS_OBJECT (object_type)' failed EXPECTED RESULT it works ADDITIONAL INFORMATION If you kill systemsettings and open the KCM again, the GTK2 theme was successfully changed, the the GTK3 one was not. I can reproduce this on git master, but a user in the Plasma telegram channel has reported it against Plasma 5.17.4 too.
Cannot reproduce anymore.
*** Bug 413492 has been marked as a duplicate of this bug. ***