fix segfalt at loadError from message with attached file
Don't delete m_itineraryModel if already deleted. coredump of the segfault:
neochat-20240712-102956.kcrash
#4 0x00007f5f20deb49e in QObject::deleteLater() () from /lib64/libQt6Core.so.6
#5 0x00000000005c23c0 in operator() (__closure=0x184aced0) at /home/andreas/kde/src/neochat/src/models/messagecontentmodel.cpp:534
#6 QtPrivate::FunctorCall<QtPrivate::IndexesList<>, QtPrivate::List<>, void, MessageContentModel::updateItineraryModel()::<lambda()> >::call (f=..., arg=<optimized out>) at /usr/include/qt6/QtCore/qobjectdefs_impl.h:137