Please see this backtrace https://bugs.archlinux.org/task/47121?getfile=13578 and this comment https://bugs.archlinux.org/task/47121#comment141101
I can confirm the same issue with SDDM, it crashes in libKF5IconThemes.so since KF 5.16 upgrade.
sddm-greeter: segfault at 0 ip 00007fc05119a371 sp 00007ffcda5c8910 error 4 in libKF5IconThemes.so.5.16.0[7fc05116c000+3f000]
This also affects me. I'm using Arch Linux and I could not start sddm until I downgraded plasma-frameworks package.
Git commit 9ccc6293f792bb5a5cae624568c0073a73064b88 by Kai Uwe Broulik.
Committed on 25/11/2015 at 17:17.
Pushed by broulik into branch 'master'.
[IconItem] Don't crash on null KIconLoader theme
M +18 -6 src/declarativeimports/core/iconitem.cpp
*** Bug 355845 has been marked as a duplicate of this bug. ***
*** This bug has been confirmed by popular vote. ***