Kubuntu 18.04
Plasma 5.12.4

Steps to Reproduce:
1. Have Dropbox installed
2. Right-click on arrow > System Tray Settings > Entries > mark Dropbox as Hidden
3. Click on Ok button

Plasmashell crashes. This is 100% reproducible for me. Sorry for the lousy backtrace. DrKonqi couldn't find anything.

(gdb) bt
#0  0x00007ffff4b15a65 in QV4::QObjectWrapper::getProperty(QV4::ExecutionEngine*, QObject*, int, bool) () at /usr/lib/x86_64-linux-gnu/libQt5Qml.so.5
#1  0x00007ffff4b2ce53 in QV4::Runtime::method_getQmlQObjectProperty(QV4::ExecutionEngine*, QV4::Value const&, int, bool) () at /usr/lib/x86_64-linux-gnu/libQt5Qml.so.5
#2  0x00007fff2f22374f in  ()
#3  0x00007fffd23a0010 in  ()
#4  0x00007fffcc03a188 in  ()
#5  0x4062c00000000000 in  ()
#6  0x0000555555a14760 in  ()
#7  0x0000000000000001 in  ()
#8  0x0000555555a14760 in  ()
#9  0x00007fffffff9170 in  ()
#10 0x00007fffd23a03c0 in  ()
#11 0x00007fffffff93f0 in  ()
#12 0x0000555555a14760 in  ()
#13 0x0000555558b12780 in  ()
#14 0x00007ffff4a9d8cc in QV4::ExecutionContext::simpleCall(QV4::Scope&, QV4::CallData*, QV4::Function*) () at /usr/lib/x86_64-linux-gnu/libQt5Qml.so.5
#15 0x00007ffff4bc6a29 in QQmlJavaScriptExpression::evaluate(QV4::CallData*, bool*, QV4::Scope&) () at /usr/lib/x86_64-linux-gnu/libQt5Qml.so.5
#16 0x00007ffff4bd0ce6 in  () at /usr/lib/x86_64-linux-gnu/libQt5Qml.so.5
#17 0x00007ffff4bcdb23 in QQmlBinding::update(QFlags<QQmlPropertyData::WriteFlag>) ()
    at /usr/lib/x86_64-linux-gnu/libQt5Qml.so.5
#18 0x00007ffff4ba3573 in QQmlNotifier::emitNotify(QQmlNotifierEndpoint*, void**) ()
    at /usr/lib/x86_64-linux-gnu/libQt5Qml.so.5
#19 0x00007ffff4b40f99 in QQmlData::signalEmitted(QAbstractDeclarativeData*, QObject*, int, void**) () at /usr/lib/x86_64-linux-gnu/libQt5Qml.so.5
#20 0x00007ffff1c85def in QMetaObject::activate(QObject*, int, int, void**) ()
    at /usr/lib/x86_64-linux-gnu/libQt5Core.so.5
#21 0x00007ffff583b869 in QQuickItem::geometryChanged(QRectF const&, QRectF const&) ()
    at /usr/lib/x86_64-linux-gnu/libQt5Quick.so.5
#22 0x00007ffff58358eb in QQuickItem::setY(double) ()
    at /usr/lib/x86_64-linux-gnu/libQt5Quick.so.5
#23 0x00007ffff5845196 in  () at /usr/lib/x86_64-linux-gnu/libQt5Quick.so.5
#24 0x00007ffff4bd076e in  () at /usr/lib/x86_64-linux-gnu/libQt5Qml.so.5
#25 0x00007ffff4bd0d7e in  () at /usr/lib/x86_64-linux-gnu/libQt5Qml.so.5
#26 0x00007ffff4bcdb23 in QQmlBinding::update(QFlags<QQmlPropertyData::WriteFlag>) ()
    at /usr/lib/x86_64-linux-gnu/libQt5Qml.so.5
#27 0x00007ffff4ba3573 in QQmlNotifier::emitNotify(QQmlNotifierEndpoint*, void**) ()
    at /usr/lib/x86_64-linux-gnu/libQt5Qml.so.5
#28 0x00007ffff4b40f99 in QQmlData::signalEmitted(QAbstractDeclarativeData*, QObject*, int, void**) () at /usr/lib/x86_64-linux-gnu/libQt5Qml.so.5
#29 0x00007ffff1c85def in QMetaObject::activate(QObject*, int, int, void**) ()
    at /usr/lib/x86_64-linux-gnu/libQt5Core.so.5
#30 0x00007ffff582e5f2 in QQuickItem::childrenRectChanged(QRectF const&) ()
    at /usr/lib/x86_64-linux-gnu/libQt5Quick.so.5
#31 0x00007ffff5832353 in  () at /usr/lib/x86_64-linux-gnu/libQt5Quick.so.5
#32 0x00007ffff583b7fd in QQuickItem::geometryChanged(QRectF const&, QRectF const&) ()
    at /usr/lib/x86_64-linux-gnu/libQt5Quick.so.5
#33 0x00007ffff5835a18 in QQuickItem::setPosition(QPointF const&) ()