Yeah, after doing some changes at the Qt5 theme, on Ubuntu 17.04, I was able to reproduce the bug. I suspect that it is some trouble at libQt5Widgets (version 5.7.1).

No idea how to fix it (or if a newer version of Qt5 would have the same issue).

(In reply to Mauro Carvalho Chehab from comment #1)
> Yeah, after doing some changes at the Qt5 theme, on Ubuntu 17.04, I was able

In time, I mean, at Plasma theme.

> to reproduce the bug. I suspect that it is some trouble at libQt5Widgets
> (version 5.7.1).
> 
> No idea how to fix it (or if a newer version of Qt5 would have the same
> issue).

I'm now running Debian 10 Buster & the above crash still happens when changing the Widget style from QtCurve to Breeze. It isn't a big problem, I just thought I'd test it again to check if this bug could be closed or not, but unfortunately  not as it still happens.

System info:

Kaffeine 2.0.15
KDE Plasma 5.14.5
KDE Frameworks 5.54.0
Qt 5.11.3
Kernel 4.19.0-5-amd64

I installed a bunch of debug packages after running find-dbgsym-packages & the crash trace is now as follows:

-------------------------------------------------------------

Application: Kaffeine (kaffeine), signal: Segmentation fault
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
[Current thread is 1 (Thread 0x7fbde80ef800 (LWP 10554))]

Thread 3 (Thread 0x7fbddd638700 (LWP 10558)):
#0  0x00007fbde55776d0 in pa_pstream_ref@plt () from /usr/lib/x86_64-linux-gnu/pulseaudio/libpulsecommon-12.2.so
#1  0x00007fbde55a77a4 in ?? () from /usr/lib/x86_64-linux-gnu/pulseaudio/libpulsecommon-12.2.so
#2  0x00007fbde55a7b79 in ?? () from /usr/lib/x86_64-linux-gnu/pulseaudio/libpulsecommon-12.2.so
#3  0x00007fbde55a83ca in ?? () from /usr/lib/x86_64-linux-gnu/pulseaudio/libpulsecommon-12.2.so
#4  0x00007fbde561cca8 in pa_mainloop_dispatch () from /lib/x86_64-linux-gnu/libpulse.so.0
#5  0x00007fbde561cf7c in pa_mainloop_iterate () from /lib/x86_64-linux-gnu/libpulse.so.0
#6  0x00007fbde561d020 in pa_mainloop_run () from /lib/x86_64-linux-gnu/libpulse.so.0
#7  0x00007fbde562b049 in ?? () from /lib/x86_64-linux-gnu/libpulse.so.0
#8  0x00007fbde55b84f8 in ?? () from /usr/lib/x86_64-linux-gnu/pulseaudio/libpulsecommon-12.2.so
#9  0x00007fbdeb6b8fa3 in start_thread (arg=<optimized out>) at pthread_create.c:486
#10 0x00007fbdec0b64cf in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:95

Thread 2 (Thread 0x7fbde486c700 (LWP 10557)):
#0  __GI___libc_read (nbytes=16, buf=0x7fbde486bb50, fd=7) at ../sysdeps/unix/sysv/linux/read.c:26
#1  __GI___libc_read (fd=7, buf=buf@entry=0x7fbde486bb50, nbytes=nbytes@entry=16) at ../sysdeps/unix/sysv/linux/read.c:24
#2  0x00007fbdea776aa0 in read (__nbytes=16, __buf=0x7fbde486bb50, __fd=<optimized out>) at /usr/include/x86_64-linux-gnu/bits/unistd.h:44
#3  g_wakeup_acknowledge (wakeup=0x5644a39cc1b0) at ../../../glib/gwakeup.c:210
#4  0x00007fbdea730c0f in g_main_context_check (context=context@entry=0x7fbdd8000bf0, max_priority=2147483647, fds=fds@entry=0x7fbdd801ade0, n_fds=n_fds@entry=1) at ../../../glib/gmain.c:3698
#5  0x00007fbdea7310e0 in g_main_context_iterate (context=context@entry=0x7fbdd8000bf0, block=block@entry=1, dispatch=dispatch@entry=1, self=<optimized out>) at ../../../glib/gmain.c:3917
#6  0x00007fbdea73125c in g_main_context_iteration (context=0x7fbdd8000bf0, may_block=may_block@entry=1) at ../../../glib/gmain.c:3981
#7  0x00007fbdec74087b in QEventDispatcherGlib::processEvents (this=0x7fbdd8000b20, flags=...) at kernel/qeventdispatcher_glib.cpp:424
#8  0x00007fbdec6ee27b in QEventLoop::exec (this=this@entry=0x7fbde486bd70, flags=..., flags@entry=...) at ../../include/QtCore/../../src/corelib/global/qflags.h:140
#9  0x00007fbdec53dec6 in QThread::exec (this=this@entry=0x7fbded7f6d60 <(anonymous namespace)::Q_QGS__q_manager::innerFunction()::holder>) at ../../include/QtCore/../../src/corelib/global/qflags.h:120
#10 0x00007fbded776545 in QDBusConnectionManager::run (this=0x7fbded7f6d60 <(anonymous namespace)::Q_QGS__q_manager::innerFunction()::holder>) at qdbusconnection.cpp:178
#11 0x00007fbdec547aa7 in QThreadPrivate::start (arg=0x7fbded7f6d60 <(anonymous namespace)::Q_QGS__q_manager::innerFunction()::holder>) at thread/qthread_unix.cpp:367
#12 0x00007fbdeb6b8fa3 in start_thread (arg=<optimized out>) at pthread_create.c:486
#13 0x00007fbdec0b64cf in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:95

Thread 1 (Thread 0x7fbde80ef800 (LWP 10554)):
[KCrash Handler]
#6  0x00007fbded06e960 in QApplication::setStyle (style=0x5644a45d3c40) at ../../include/QtCore/../../src/corelib/global/qflags.h:145
#7  QApplication::setStyle (style=0x5644a45d3c40) at kernel/qapplication.cpp:1123
#8  0x00007fbded06f3c6 in QApplication::setStyle (style=...) at kernel/qapplication.cpp:1238
#9  0x00007fbde645501e in ?? () from /usr/lib/x86_64-linux-gnu/qt5/plugins/platformthemes/KDEPlasmaPlatformTheme.so
#10 0x00007fbde6463423 in ?? () from /usr/lib/x86_64-linux-gnu/qt5/plugins/platformthemes/KDEPlasmaPlatformTheme.so
#11 0x00007fbded781d8a in QDBusConnectionPrivate::deliverCall (this=<optimized out>, object=<optimized out>, msg=..., metaTypes=..., slotIdx=<optimized out>) at qdbusintegrator.cpp:991
#12 0x00007fbdec719182 in QObject::event (this=0x5644a39b6520, e=<optimized out>) at kernel/qobject.cpp:1251
#13 0x00007fbded06a4b1 in QApplicationPrivate::notify_helper (this=this@entry=0x5644a39691e0, receiver=receiver@entry=0x5644a39b6520, e=e@entry=0x5644a39d49d0) at kernel/qapplication.cpp:3726
#14 0x00007fbded071950 in QApplication::notify (this=0x7ffd1a552af0, receiver=0x5644a39b6520, e=0x5644a39d49d0) at kernel/qapplication.cpp:3485
#15 0x00007fbdec6ef5a9 in QCoreApplication::notifyInternal2 (receiver=0x5644a39b6520, event=0x5644a39d49d0) at ../../include/QtCore/5.11.3/QtCore/private/../../../../../src/corelib/thread/qthread_p.h:307
#16 0x00007fbdec6f259b in QCoreApplication::sendEvent (event=0x5644a39d49d0, receiver=<optimized out>) at ../../include/QtCore/../../src/corelib/kernel/qcoreapplication.h:234
#17 QCoreApplicationPrivate::sendPostedEvents (receiver=0x0, event_type=0, data=0x5644a3966ae0) at kernel/qcoreapplication.cpp:1744
#18 0x00007fbdec741233 in postEventSourceDispatch (s=0x5644a39b9fb0) at kernel/qeventdispatcher_glib.cpp:276
#19 0x00007fbdea730f2e in g_main_dispatch (context=0x7fbde0004ff0) at ../../../glib/gmain.c:3182
#20 g_main_context_dispatch (context=context@entry=0x7fbde0004ff0) at ../../../glib/gmain.c:3847
#21 0x00007fbdea7311c8 in g_main_context_iterate (context=context@entry=0x7fbde0004ff0, block=block@entry=1, dispatch=dispatch@entry=1, self=<optimized out>) at ../../../glib/gmain.c:3920
#22 0x00007fbdea73125c in g_main_context_iteration (context=0x7fbde0004ff0, may_block=may_block@entry=1) at ../../../glib/gmain.c:3981
#23 0x00007fbdec740863 in QEventDispatcherGlib::processEvents (this=0x5644a39b8600, flags=...) at kernel/qeventdispatcher_glib.cpp:422
#24 0x00007fbde7d3f3e1 in QPAEventDispatcherGlib::processEvents (this=0x5644a39b8600, flags=...) at qeventdispatcher_glib.cpp:69
#25 0x00007fbdec6ee27b in QEventLoop::exec (this=this@entry=0x7ffd1a552a30, flags=..., flags@entry=...) at ../../include/QtCore/../../src/corelib/global/qflags.h:140
#26 0x00007fbdec6f6262 in QCoreApplication::exec () at ../../include/QtCore/../../src/corelib/global/qflags.h:120
#27 0x00005644a35ac6fb in main (argc=<optimized out>, argv=<optimized out>) at ./src/main.cpp:228
[Inferior 1 (process 10554) detached]

KDE doesn't have a 'Widget Style' bit in system settings any more that I can find. I tried changing a few different desktop appearance related things; themes, application styles, window decorations & colours, and Kaffeine didn't crash, so I'll closed this & mark it as resolved.