This apparently happened during a resume cycle:

(gdb) bt
#0  0x00007ff0d62be21b in __GI_raise (sig=sig@entry=6) at ../sysdeps/unix/sysv/linux/raise.c:54
#1  0x00007ff0d62bf8b1 in __GI_abort () at abort.c:89
#2  0x00007ff0d701a99e in qt_message_fatal (context=..., message=<synthetic pointer>...)
    at /var/tmp/portage/dev-qt/qtcore-5.9.9999/work/qtcore-5.9.9999/src/corelib/global/qlogging.cpp:1690
#3  QMessageLogger::fatal (this=this@entry=0x7fff92d751d0, msg=msg@entry=0x7ff0d72df038 "ASSERT: \"%s\" in file %s, line %d")
    at /var/tmp/portage/dev-qt/qtcore-5.9.9999/work/qtcore-5.9.9999/src/corelib/global/qlogging.cpp:796
#4  0x00007ff0d7014c45 in qt_assert (assertion=<optimized out>, file=<optimized out>, line=<optimized out>)
    at /var/tmp/portage/dev-qt/qtcore-5.9.9999/work/qtcore-5.9.9999/src/corelib/global/qglobal.cpp:3077
#5  0x0000000000535e50 in QtPrivate::QFunctorSlotObject<Cryptography::TopLevelMessage::fetchChildren(Cryptography::MessageModel*)::$_0, 1, QtPrivate::List<QModelIndex const&>, void>::impl(int, QtPrivate::QSlotObjectBase*, QObject*, void**, bool*) ()
#6  0x00007ff0d7246505 in QtPrivate::QSlotObjectBase::call (a=0x7fff92d75370, r=0x1ea9a10, this=0x303b760)
    at ../../include/QtCore/../../../qtcore-5.9.9999/src/corelib/kernel/qobject_impl.h:101
#7  QMetaObject::activate (sender=sender@entry=0x1ab5a00, signalOffset=<optimized out>, local_signal_index=local_signal_index@entry=10, argv=argv@entry=0x7fff92d75370)
    at /var/tmp/portage/dev-qt/qtcore-5.9.9999/work/qtcore-5.9.9999/src/corelib/kernel/qobject.cpp:3749
#8  0x00007ff0d7246aa5 in QMetaObject::activate (sender=sender@entry=0x1ab5a00, m=m@entry=0x7ff0d7474540 <QAbstractItemModel::staticMetaObject>, 
    local_signal_index=local_signal_index@entry=10, argv=argv@entry=0x7fff92d75370)
    at /var/tmp/portage/dev-qt/qtcore-5.9.9999/work/qtcore-5.9.9999/src/corelib/kernel/qobject.cpp:3628
#9  0x00007ff0d71b7ccc in QAbstractItemModel::rowsInserted (this=this@entry=0x1ab5a00, _t1=..., _t2=1, _t3=11, _t4=...) at .moc/moc_qabstractitemmodel.cpp:603
#10 0x00007ff0d71c102c in QAbstractItemModel::endInsertRows (this=0x1ab5a00)
    at /var/tmp/portage/dev-qt/qtcore-5.9.9999/work/qtcore-5.9.9999/src/corelib/itemmodels/qabstractitemmodel.cpp:2709
#11 0x00000000005c18b2 in Imap::Mailbox::Model::replaceChildMailboxes(Imap::Mailbox::TreeItemMailbox*, QVector<Imap::Mailbox::TreeItem*> const&) ()
#12 0x00000000005c0f03 in Imap::Mailbox::Model::finalizeList(Imap::Parser*, Imap::Mailbox::TreeItemMailbox*) ()
#13 0x000000000061116e in Imap::Mailbox::ListChildMailboxesTask::handleStateHelper(Imap::Responses::State const*) ()
#14 0x00000000005bd545 in Imap::Mailbox::Model::responseReceived(QMap<Imap::Parser*, Imap::Mailbox::ParserState>::iterator) ()
#15 0x00007ff0d72474da in QObject::event (this=0x1ab5a00, e=<optimized out>)
    at /var/tmp/portage/dev-qt/qtcore-5.9.9999/work/qtcore-5.9.9999/src/corelib/kernel/qobject.cpp:1246
#16 0x00007ff0d7b03d54 in QApplicationPrivate::notify_helper (this=<optimized out>, receiver=0x1ab5a00, e=0x26e4d80)
    at /var/tmp/portage/dev-qt/qtwidgets-5.9.9999/work/qtwidgets-5.9.9999/src/widgets/kernel/qapplication.cpp:3722
#17 0x00007ff0d7b0c4a2 in QApplication::notify (this=0x7fff92d75d90, receiver=0x1ab5a00, e=0x26e4d80)
    at /var/tmp/portage/dev-qt/qtwidgets-5.9.9999/work/qtwidgets-5.9.9999/src/widgets/kernel/qapplication.cpp:3481
#18 0x00007ff0d7216078 in QCoreApplication::notifyInternal2 (receiver=0x1ab5a00, event=event@entry=0x26e4d80)
    at /var/tmp/portage/dev-qt/qtcore-5.9.9999/work/qtcore-5.9.9999/src/corelib/kernel/qcoreapplication.cpp:1018
#19 0x00007ff0d7219237 in QCoreApplication::sendEvent (event=0x26e4d80, receiver=<optimized out>)
    at /var/tmp/portage/dev-qt/qtcore-5.9.9999/work/qtcore-5.9.9999/src/corelib/kernel/qcoreapplication.h:233
#20 QCoreApplicationPrivate::sendPostedEvents (receiver=receiver@entry=0x0, event_type=event_type@entry=0, data=0x18b87b0)
    at /var/tmp/portage/dev-qt/qtcore-5.9.9999/work/qtcore-5.9.9999/src/corelib/kernel/qcoreapplication.cpp:1678
#21 0x00007ff0d721980e in QCoreApplication::sendPostedEvents (receiver=receiver@entry=0x0, event_type=event_type@entry=0)
    at /var/tmp/portage/dev-qt/qtcore-5.9.9999/work/qtcore-5.9.9999/src/corelib/kernel/qcoreapplication.cpp:1532
#22 0x00007ff0d7273369 in postEventSourceDispatch (s=0x1917300)
    at /var/tmp/portage/dev-qt/qtcore-5.9.9999/work/qtcore-5.9.9999/src/corelib/kernel/qeventdispatcher_glib.cpp:276
#23 0x00007ff0d186c80c in g_main_dispatch (context=0x7ff0c40016f0) at /var/tmp/portage/dev-libs/glib-2.48.2/work/glib-2.48.2/glib/gmain.c:3154
#24 g_main_context_dispatch (context=context@entry=0x7ff0c40016f0) at /var/tmp/portage/dev-libs/glib-2.48.2/work/glib-2.48.2/glib/gmain.c:3769
#25 0x00007ff0d186cb18 in g_main_context_iterate (context=context@entry=0x7ff0c40016f0, block=block@entry=1, dispatch=dispatch@entry=1, self=<optimized out>)
    at /var/tmp/portage/dev-libs/glib-2.48.2/work/glib-2.48.2/glib/gmain.c:3840
#26 0x00007ff0d186cc21 in g_main_context_iteration (context=0x7ff0c40016f0, may_block=1) at /var/tmp/portage/dev-libs/glib-2.48.2/work/glib-2.48.2/glib/gmain.c:3901
#27 0x00007ff0d7272e9d in QEventDispatcherGlib::processEvents (this=0x1912c60, flags=...)
    at /var/tmp/portage/dev-qt/qtcore-5.9.9999/work/qtcore-5.9.9999/src/corelib/kernel/qeventdispatcher_glib.cpp:423
#28 0x00007ff0d72141ba in QEventLoop::exec (this=this@entry=0x7fff92d75be0, flags=..., flags@entry=...)
    at /var/tmp/portage/dev-qt/qtcore-5.9.9999/work/qtcore-5.9.9999/src/corelib/kernel/qeventloop.cpp:212
#29 0x00007ff0d721dd48 in QCoreApplication::exec () at /var/tmp/portage/dev-qt/qtcore-5.9.9999/work/qtcore-5.9.9999/src/corelib/kernel/qcoreapplication.cpp:1291
#30 0x0000000000454e08 in main ()