After checking out the new vault feature, I decided I didn't want the icon showing all the time in the systray and used the settings to hide it. After closing the systray settings, the icon was still showing for a second and then plasmashell crashed.

The backtrace seems to implicate kf5windowsystem and QML though:

Thread 1 (Thread 0x7f3fb2cdb8c0 (LWP 3865)):
[KCrash Handler]
#6  QVector<QQmlPropertyData>::count (this=<optimized out>) at /usr/include/x86_64-linux-gnu/qt5/QtCore/qvector.h:234
#7  QQmlPropertyCache::property (index=5, this=0x0) at ../../include/QtQml/5.9.1/QtQml/private/../../../../../src/qml/qml/qqmlpropertycache_p.h:711
#8  QV4::QObjectWrapper::getProperty (engine=0x1baba30, object=0x4460950, propertyIndex=5, captureRequired=true) at jsruntime/qv4qobjectwrapper.cpp:567
#9  0x00007f3fb023b65a in QV4::Runtime::method_getQmlQObjectProperty (engine=0x1baba30, object=..., propertyIndex=<optimized out>, captureRequired=<optimized out>) at jsruntime/qv4runtime.cpp:1491
#10 0x00007f3eeb4dc17b in ?? ()
#11 0xffffffffffffffff in ?? ()
#12 0x00007ffd2b44ed50 in ?? ()
#13 0x00007ffd2b44ebb0 in ?? ()
#14 0xaa39325d0fd15700 in ?? ()
#15 0x000000000397de80 in ?? ()
#16 0x00007f3f971d4a54 in KWindowInfoPrivateX11::state (this=0x3a61fb0) at /workspace/build/src/platforms/xcb/kwindowinfo.cpp:102
#17 0xaa39325d0fd15700 in ?? ()
#18 0x0000000000000000 in ?? ()