Fix crash when starting konsole

valgrind says
==31368== Invalid read of size 8
==31368==    at 0x2801D880: QList<QPlatformMenuItem*>::QList(QList<QPlatformMenuItem*> const&) (qlist.h:803)
==31368==    by 0x2801A5BC: UbuntuPlatformMenu::menuItems() const (gmenumodelplatformmenu.cpp:361)
==31368==    by 0x28011257: UbuntuGMenuModelExporter::addSubmenuItems(UbuntuPlatformMenu*, _GMenu*) (gmenumodelexporter.cpp:228)
==31368==    by 0x28011537: UbuntuGMenuModelExporter::createSubmenu(QPlatformMenu*, UbuntuPlatformMenuItem*) (gmenumodelexporter.cpp:217)
==31368==    by 0x28011164: UbuntuGMenuModelExporter::processItemForGMenu(QPlatformMenuItem*, _GMenu*) (gmenumodelexporter.cpp:296)
==31368==    by 0x28011350: UbuntuGMenuModelExporter::addSubmenuItems(UbuntuPlatformMenu*, _GMenu*) (gmenumodelexporter.cpp:244)
==31368==    by 0x28011537: UbuntuGMenuModelExporter::createSubmenu(QPlatformMenu*, UbuntuPlatformMenuItem*) (gmenumodelexporter.cpp:217)
==31368==    by 0x28011BB6: operator() (gmenumodelexporter.cpp:69)
==31368==    by 0x28011BB6: call (qobjectdefs_impl.h:135)
==31368==    by 0x28011BB6: call<QtPrivate::List<>, void> (qobjectdefs_impl.h:192)
==31368==    by 0x28011BB6: QtPrivate::QFunctorSlotObject<UbuntuMenuBarExporter::UbuntuMenuBarExporter(UbuntuPlatformMenuBar*)::{lambda()#2}, 0, QtPrivate::List<>, void>::impl(int, QtPrivate::QSlotObjectBase*, QObject*, void**, bool*) (qobject_impl.h:169)
==31368==    by 0x8BE581D: call (qobject_impl.h:101)
==31368==    by 0x8BE581D: QMetaObject::activate(QObject*, int, int, void**) (qobject.cpp:3723)
==31368==    by 0x8BF1E37: QTimer::timerEvent(QTimerEvent*) (qtimer.cpp:254)
==31368==    by 0x8BE6312: QObject::event(QEvent*) (qobject.cpp:1285)
==31368==    by 0x786A35B: QApplicationPrivate::notify_helper(QObject*, QEvent*) (qapplication.cpp:3799)
==31368==  Address 0x26408048 is 56 bytes inside a block of size 88 free'd
==31368==    at 0x4C2E25B: operator delete(void*) (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==31368==    by 0x79E11C1: QMenuPrivate::~QMenuPrivate() (qmenu_p.h:297)
==31368==    by 0x79E1248: QMenuPrivate::~QMenuPrivate() (qmenu_p.h:301)
==31368==    by 0x8BED0A6: cleanup (qscopedpointer.h:60)
==31368==    by 0x8BED0A6: ~QScopedPointer (qscopedpointer.h:107)
==31368==    by 0x8BED0A6: QObject::~QObject() (qobject.cpp:900)
==31368==    by 0x78AD452: QWidget::~QWidget() (qwidget.cpp:1555)
==31368==    by 0x79DA918: QMenu::~QMenu() (qmenu.cpp:1518)
==31368==    by 0x8BE638F: QObject::event(QEvent*) (qobject.cpp:1254)
==31368==    by 0x78B1C6A: QWidget::event(QEvent*) (qwidget.cpp:9225)
==31368==    by 0x79E0342: QMenu::event(QEvent*) (qmenu.cpp:2799)
==31368==    by 0x786A35B: QApplicationPrivate::notify_helper(QObject*, QEvent*) (qapplication.cpp:3799)
==31368==    by 0x7871B10: QApplication::notify(QObject*, QEvent*) (qapplication.cpp:3556)
==31368==    by 0x8BB989F: QCoreApplication::notifyInternal2(QObject*, QEvent*) (qcoreapplication.cpp:988)
==31368==  Block was alloc'd at
==31368==    at 0x4C2D19F: operator new(unsigned long) (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==31368==    by 0x2800F6EB: UbuntuAppMenuTheme::createPlatformMenu() const (theme.cpp:69)
==31368==    by 0x79D512D: QMenuPrivate::init() (qmenu.cpp:156)
==31368==    by 0x6DE8354: KActionMenu::KActionMenu(QIcon const&, QString const&, QObject*) (in /usr/lib/x86_64-linux-gnu/libKF5WidgetsAddons.so.5.28.0)
==31368==    by 0x4E56833: ??? (in /usr/lib/x86_64-linux-gnu/libkdeinit5_konsole.so)
==31368==    by 0x4E591F2: ??? (in /usr/lib/x86_64-linux-gnu/libkdeinit5_konsole.so)
==31368==    by 0x4E4D90F: ??? (in /usr/lib/x86_64-linux-gnu/libkdeinit5_konsole.so)
==31368==    by 0x4E4E373: ??? (in /usr/lib/x86_64-linux-gnu/libkdeinit5_konsole.so)
==31368==    by 0x4E528F1: ??? (in /usr/lib/x86_64-linux-gnu/libkdeinit5_konsole.so)
==31368==    by 0x4E60D24: kdemain (in /usr/lib/x86_64-linux-gnu/libkdeinit5_konsole.so)
==31368==    by 0x50A03F0: (below main) (libc-start.c:291)