Reported downstream and reproducible:

1 - Select a message,
2 - RMB / Add note
3 - Unrelated to the crash, but the default note text is "AnnotationEditDialogPrivate note"
4 - Type / OK

#0  0x00007f50ef1973db in free () at /usr/lib64/libc.so.6
#1  0x00007f50edc24622 in PimCommon::AnnotationEditDialog::~AnnotationEditDialog() (this=0x7ffc1739b480)
    at /data/kde/src/pimcommon/src/pimcommonakonadi/widgets/annotationdialog.cpp:103
#2  0x00007f50f090695a in QDialog::exec() () at /usr/lib64/libQt5Widgets.so.5
#3  0x00007f50facd2957 in KMail::MessageActions::annotateMessage() (this=0x4a3f900) at /data/kde/src/kmail/src/messageactions.cpp:702
#4  0x00007f50facd7b1b in QtPrivate::FunctorCall<QtPrivate::IndexesList<>, QtPrivate::List<>, void, void (KMail::MessageActions::*)()>::call(void (KMail::MessageActions::*)(), KMail::MessageActions*, void**)
    (f=(void (KMail::MessageActions::*)(class KMail::MessageActions * const)) 0x7f50facd28f0 <KMail::MessageActions::annotateMessage()>, o=0x4a3f900, arg=0x7ffc1739b6c0)
    at /usr/include/qt5/QtCore/qobjectdefs_impl.h:152
#5  0x00007f50facd7a8d in QtPrivate::FunctionPointer<void (KMail::MessageActions::*)()>::call<QtPrivate::List<>, void>(void (KMail::MessageActions::*)(), KMail::MessageActions*, void**)
    (f=(void (KMail::MessageActions::*)(class KMail::MessageActions * const)) 0x7f50facd28f0 <KMail::MessageActions::annotateMessage()>, o=0x4a3f900, arg=0x7ffc1739b6c0)
    at /usr/include/qt5/QtCore/qobjectdefs_impl.h:185
#6  0x00007f50facd79d2 in QtPrivate::QSlotObject<void (KMail::MessageActions::*)(), QtPrivate::List<>, void>::impl(int, QtPrivate::QSlotObjectBase*, QObject*, void**, bool*)
    (which=1, this_=0x4a87fd0, r=0x4a3f900, a=0x7ffc1739b6c0, ret=0x0) at /usr/include/qt5/QtCore/qobjectdefs_impl.h:418
#7  0x00007f50ef9384f3 in QtPrivate::QSlotObjectBase::destroyIfLastRef() (this=0x4a87fd0) at ../../include/QtCore/../../src/corelib/kernel/qobjectdefs_impl.h:395
#8  QtPrivate::QSlotObjectBase::destroyIfLastRef() (this=0x4a87fd0) at ../../include/QtCore/../../src/corelib/kernel/qobjectdefs_impl.h:394
#9  doActivate<false>(QObject*, int, void**)::Deleter::operator()(QtPrivate::QSlotObjectBase*) const (slot=0x4a87fd0, this=<optimized out>) at kernel/qobject.cpp:3879
#10 std::unique_ptr<QtPrivate::QSlotObjectBase, doActivate<false>(QObject*, int, void**)::Deleter>::~unique_ptr() (this=<synthetic pointer>, __in_chrg=<optimized out>)
    at /usr/include/c++/11/bits/unique_ptr.h:361
#11 doActivate<false>(QObject*, int, void**) (sender=0x4a84920, signal_index=4, argv=0x7ffc1739b6c0) at kernel/qobject.cpp:3888
#12 0x00007f50f06f6182 in QAction::triggered(bool) () at /usr/lib64/libQt5Widgets.so.5
#13 0x00007f50f06f8db4 in QAction::activate(QAction::ActionEvent) () at /usr/lib64/libQt5Widgets.so.5
#14 0x00007f50f087ea52 in  () at /usr/lib64/libQt5Widgets.so.5
#15 0x00007f50f0886841 in  () at /usr/lib64/libQt5Widgets.so.5