SUMMARY
After this upstream commit (which is not in 5.15.2, but is in the KDE maintenance branch and, presumably, in the commercial 5.15.3), kbibtex crashes immediately at startup

https://code.qt.io/cgit/qt/qtbase.git/commit/?h=5.15&id=4796296ed6d7b05e2ba94e5a8a28c48350a3c492

Backtrace:

(gdb) bt
#0  0x00007ffff77bf75c in FileModel::element(int) const (this=0x0, row=0) at /usr/src/debug/kbibtex-0.9.2/src/data/models/filemodel.cpp:399
#1  0x00007ffff7f14f78 in SortFilterFileModel::filterAcceptsRow(int, QModelIndex const&) const
    (this=0x55555803cbe0, source_row=<optimized out>, source_parent=<optimized out>)
    at /usr/src/debug/kbibtex-0.9.2/src/gui/file/sortfilterfilemodel.cpp:151
#2  0x00007ffff62d95f7 in QSortFilterProxyModelPrivate::filterAcceptsRowInternal(int, QModelIndex const&) const
    (this=this@entry=0x555557eae3c0, source_row=<optimized out>, source_parent=...) at itemmodels/qsortfilterproxymodel.cpp:446
#3  0x00007ffff62dd4aa in QSortFilterProxyModelPrivate::create_mapping(QModelIndex const&) const (this=this@entry=0x555557eae3c0, source_parent=...)
    at itemmodels/qsortfilterproxymodel.cpp:504
#4  0x00007ffff62e4a0c in QSortFilterProxyModel::setSourceModel(QAbstractItemModel*) (this=0x55555803cbe0, sourceModel=0x7fffffffd700)
    at itemmodels/qsortfilterproxymodel.cpp:2149
#5  0x00007ffff7f16012 in SortFilterFileModel::setSourceModel(QAbstractItemModel*) (this=this@entry=0x55555803cbe0, model=0x555557eaf190)
    at /usr/src/debug/kbibtex-0.9.2/src/gui/file/sortfilterfilemodel.cpp:43
#6  0x00007fffc4015543 in KBibTeXPart::KBibTeXPartPrivate::openFile(QUrl const&, QString const&) (this=this@entry=
    0x555557dd8310, url=..., localFilePath=...) at /usr/src/debug/kbibtex-0.9.2/src/parts/part.cpp:391
#7  0x00007fffc400e1e2 in KBibTeXPart::openFile() (this=0x555557e4d430) at /usr/src/debug/kbibtex-0.9.2/src/parts/part.cpp:964
#8  0x00007ffff7c4a315 in  () at /usr/lib/libKF5Parts.so.5
#9  0x00007ffff7c4b0e4 in KParts::ReadOnlyPart::openUrl(QUrl const&) () at /usr/lib/libKF5Parts.so.5
#10 0x000055555559cb54 in OpenFileInfo::OpenFileInfoPrivate::createPart(QWidget*, QExplicitlySharedDataPointer<KService>)
    (this=0x555555b839c0, newWidgetParent=0x555555a791a0, newServicePtr=...) at /usr/src/debug/kbibtex-0.9.2/src/program/openfileinfo.cpp:139
#11 0x000055555559bd67 in OpenFileInfo::part(QWidget*, QExplicitlySharedDataPointer<KService>)
    (this=this@entry=0x555555b844c0, parent=parent@entry=0x555555a791a0, servicePtr=...) at /usr/src/debug/kbibtex-0.9.2/src/program/openfileinfo.cpp:263
#12 0x00005555555809cf in MDIWidget::setFile(OpenFileInfo*, QExplicitlySharedDataPointer<KService>)
    (this=0x555555a791a0, openFileInfo=0x555555b844c0, servicePtr=...) at /usr/src/debug/kbibtex-0.9.2/src/program/mdiwidget.cpp:242
#13 0x000055555557c766 in QtPrivate::FunctorCall<QtPrivate::IndexesList<0, 1>, QtPrivate::List<OpenFileInfo*, QExplicitlySharedDataPointer<KService> >, void, void (MDIWidget::*)(OpenFileInfo*, QExplicitlySharedDataPointer<KService>)>::call(void (MDIWidget::*)(OpenFileInfo*, QExplicitlySharedDataPointer<KSer--Type <RET> for more, q to quit, c to continue without paging--
vice>), MDIWidget*, void**) (arg=<optimized out>, o=<optimized out>, f=<optimized out>) at /usr/include/qt/QtCore/qobjectdefs_impl.h:152
#14 QtPrivate::FunctionPointer<void (MDIWidget::*)(OpenFileInfo*, QExplicitlySharedDataPointer<KService>)>::call<QtPrivate::List<OpenFileInfo*, QExplicitlySharedDataPointer<KService> >, void>(void (MDIWidget::*)(OpenFileInfo*, QExplicitlySharedDataPointer<KService>), MDIWidget*, void**)
    (arg=<optimized out>, o=<optimized out>, f=<optimized out>) at /usr/include/qt/QtCore/qobjectdefs_impl.h:185
#15 QtPrivate::QSlotObject<void (MDIWidget::*)(OpenFileInfo*, QExplicitlySharedDataPointer<KService>), QtPrivate::List<OpenFileInfo*, QExplicitlySharedDataPointer<KService> >, void>::impl(int, QtPrivate::QSlotObjectBase*, QObject*, void**, bool*)
    (which=<optimized out>, this_=<optimized out>, r=<optimized out>, a=<optimized out>, ret=<optimized out>)
    at /usr/include/qt/QtCore/qobjectdefs_impl.h:418
#16 0x00007ffff6349a0c in QtPrivate::QSlotObjectBase::call(QObject*, void**) (a=0x7fffffffdc50, r=0x555555a791a0, this=0x555557ddc120)
    at ../../include/QtCore/../../src/corelib/kernel/qobjectdefs_impl.h:398
#17 doActivate<false>(QObject*, int, void**) (sender=0x555555c3cdb0, signal_index=3, argv=0x7fffffffdc50) at kernel/qobject.cpp:3886
#18 0x00007ffff6340df5 in QMetaObject::activate(QObject*, QMetaObject const*, int, void**)
    (sender=sender@entry=0x555555c3cdb0, m=m@entry=0x5555555b9aa0 <OpenFileInfoManager::staticMetaObject>, local_signal_index=local_signal_index@entry=0, argv=argv@entry=0x7fffffffdc50) at kernel/qobject.cpp:3946
#19 0x0000555555574cd5 in OpenFileInfoManager::currentChanged(OpenFileInfo*, QExplicitlySharedDataPointer<KService>)
    (this=this@entry=0x555555c3cdb0, _t1=<optimized out>, _t1@entry=0x555555b844c0, _t2=...)
    at /usr/src/debug/build/src/program/kbibtex_autogen/EWIEGA46WW/moc_openfileinfo.cpp:280
#20 0x0000555555599d61 in OpenFileInfoManager::setCurrentFile(OpenFileInfo*, QExplicitlySharedDataPointer<KService>)
    (this=0x555555c3cdb0, openFileInfo=openFileInfo@entry=0x555555b844c0, servicePtr=...)
    at /usr/src/debug/kbibtex-0.9.2/src/program/openfileinfo.cpp:714
#21 0x000055555559a04e in operator() (__closure=0x7fffffffdcf0) at /usr/include/qt/QtCore/qshareddata.h:183
#22 QtPrivate::FunctorCall<QtPrivate::IndexesList<>, QtPrivate::List<>, void, OpenFileInfoManager::OpenFileInfoManager(QObject*)::<lambda()> >::call
    (arg=<optimized out>, f=...) at /usr/include/qt/QtCore/qobjectdefs_impl.h:146
#23 QtPrivate::Functor<OpenFileInfoManager::OpenFileInfoManager(QObject*)::<lambda()>, 0>::call<QtPrivate::List<>, void> (arg=<optimized out>, f=...)
    at /usr/include/qt/QtCore/qobjectdefs_impl.h:256
--Type <RET> for more, q to quit, c to continue without paging--
#24 QtPrivate::QFunctorSlotObject<OpenFileInfoManager::OpenFileInfoManager(QObject*)::<lambda()>, 0, QtPrivate::List<>, void>::impl(int, QtPrivate::QSlotObjectBase *, QObject *, void **, bool *) (which=<optimized out>, this_=0x7fffffffdce0, r=<optimized out>, a=<optimized out>, ret=<optimized out>)
    at /usr/include/qt/QtCore/qobjectdefs_impl.h:443
#25 0x00007ffff634f9c3 in QtPrivate::QSlotObjectBase::call(QObject*, void**) (a=0x7fffffffdd30, r=<optimized out>, this=<optimized out>)
    at ../../include/QtCore/../../src/corelib/kernel/qobjectdefs_impl.h:398
#26 QSingleShotTimer::timerEvent(QTimerEvent*) (this=0x555555a803d0) at kernel/qtimer.cpp:320
#27 0x00007ffff634185d in QObject::event(QEvent*) (this=0x555555a803d0, e=0x7fffffffe080) at kernel/qobject.cpp:1291
#28 0x00007ffff6dc9c26 in QApplicationPrivate::notify_helper(QObject*, QEvent*) (this=this@entry=
    0x5555555bf000, receiver=receiver@entry=0x555555a803d0, e=e@entry=0x7fffffffe080) at kernel/qapplication.cpp:3632
#29 0x00007ffff6dd24ee in QApplication::notify(QObject*, QEvent*) (this=0x7fffffffe3d0, receiver=0x555555a803d0, e=0x7fffffffe080)
    at kernel/qapplication.cpp:2972
#30 0x00007ffff6307338 in QCoreApplication::notifyInternal2(QObject*, QEvent*) (receiver=0x555555a803d0, event=0x7fffffffe080)
    at kernel/qcoreapplication.cpp:1064
#31 0x00007ffff630754c in QCoreApplication::sendEvent(QObject*, QEvent*) (receiver=<optimized out>, event=<optimized out>)
    at kernel/qcoreapplication.cpp:1462
#32 0x00007ffff63709d3 in QTimerInfoList::activateTimers() (this=0x55555561df50) at kernel/qtimerinfo_unix.cpp:643
#33 0x00007ffff637121a in timerSourceDispatch(GSource*, GSourceFunc, gpointer) (source=<optimized out>) at kernel/qeventdispatcher_glib.cpp:183
#34 0x00007fffea3b4f9c in g_main_context_dispatch () at /usr/lib/libglib-2.0.so.0
#35 0x00007fffea408a49 in  () at /usr/lib/libglib-2.0.so.0
#36 0x00007fffea3b26f1 in g_main_context_iteration () at /usr/lib/libglib-2.0.so.0
#37 0x00007ffff63715b4 in QEventDispatcherGlib::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) (this=0x5555556ff670, flags=...)
    at kernel/qeventdispatcher_glib.cpp:423
#38 0x00007fffe2d61e06 in QXcbGlibEventDispatcher::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) (this=<optimized out>, flags=...)
    at qxcbeventdispatcher.cpp:143
#39 0x00007ffff6304df1 in QEventLoop::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) (this=this@entry=0x7fffffffe2e0, flags=..., flags@entry=...)
--Type <RET> for more, q to quit, c to continue without paging--
    at kernel/qeventloop.cpp:139
#40 0x00007ffff6305435 in QEventLoop::exec(QFlags<QEventLoop::ProcessEventsFlag>) (this=this@entry=0x7fffffffe2e0, flags=flags@entry=...)
    at ../../include/QtCore/../../src/corelib/global/qflags.h:69
#41 0x00007ffff630fa2e in QCoreApplication::exec() () at ../../include/QtCore/../../src/corelib/global/qflags.h:121
#42 0x00007ffff66dabbe in QGuiApplication::exec() () at kernel/qguiapplication.cpp:1867
#43 0x00007ffff6dc9b9b in QApplication::exec() () at kernel/qapplication.cpp:2824
#44 0x00005555555734a3 in main(int, char**) (argc=<optimized out>, argv=<optimized out>) at /usr/src/debug/kbibtex-0.9.2/src/program/program.cpp:124