The Model must be destroyed after the iterator, otherwise
the iterator's d'tor will cause memory corruption trying to lock
the model:
```
Invalid read of size 4
at pthread_rwlock_unlock (pthread_rwlock_unlock.c:39)
by Glib::Threads::RWLock::ReaderLock::~ReaderLock() (threads.h:828)
by void boost::checked_delete<Glib::Threads::RWLock::ReaderLock>(Glib::Threads::RWLock::ReaderLock*) (checked_delete.hpp:36)
by boost::detail::sp_counted_impl_p<Glib::Threads::RWLock::ReaderLock>::dispose() (sp_counted_impl.hpp:89)
by boost::detail::sp_counted_base::release() (sp_counted_base_gcc_atomic.hpp:120)
by boost::detail::shared_count::~shared_count() (shared_count.hpp:432)
by boost::shared_ptr<Glib::Threads::RWLock::ReaderLock>::~shared_ptr() (shared_ptr.hpp:335)
by Evoral::Sequence<Temporal::Beats>::const_iterator::~const_iterator() (Sequence.h:224)
by ARDOUR::MIDITrigger::~MIDITrigger() (triggerbox.cc:2037)
by ARDOUR::MIDITrigger::~MIDITrigger() (triggerbox.cc:2039)
by ARDOUR::TriggerBoxThread::delete_trigger(ARDOUR::Trigger*) (triggerbox.cc:4386)
by ARDOUR::TriggerBoxThread::thread_work() (triggerbox.cc:4314)
by ARDOUR::TriggerBoxThread::_thread_work(void*) (triggerbox.cc:4285)
by fake_thread_start(void*) (pthread_utils.cc:101)
by start_thread (pthread_create.c:477)
by clone (clone.S:95)
Address 0x28229798 is 24 bytes inside a block of size 56 free'd
at free (in /usr/lib/x86_64-linux-gnu/valgrind/vgpreload_memcheck-amd64-linux.so)
by Glib::Threads::RWLock::~RWLock() (in /usr/lib/x86_64-linux-gnu/libglibmm-2.4.so.1.3.0)
by Evoral::Sequence<Temporal::Beats>::~Sequence() (Sequence.h:68)
by ARDOUR::AutomatableSequence<Temporal::Beats>::~AutomatableSequence() (automatable_sequence.h:31)
by ARDOUR::MidiModel::~MidiModel() (midi_model.h:58)
by ARDOUR::MidiModel::~MidiModel() (midi_model.h:58)
by void boost::checked_delete<ARDOUR::MidiModel>(ARDOUR::MidiModel*) (checked_delete.hpp:36)
by boost::detail::sp_counted_impl_p<ARDOUR::MidiModel>::dispose() (sp_counted_impl.hpp:89)
by boost::detail::sp_counted_base::release() (sp_counted_base_gcc_atomic.hpp:120)
by boost::detail::shared_count::~shared_count() (shared_count.hpp:432)
by boost::shared_ptr<ARDOUR::MidiModel>::~shared_ptr() (shared_ptr.hpp:335)
by ARDOUR::MIDITrigger::~MIDITrigger() (triggerbox.cc:2037)
by ARDOUR::MIDITrigger::~MIDITrigger() (triggerbox.cc:2039)
by ARDOUR::TriggerBoxThread::delete_trigger(ARDOUR::Trigger*) (triggerbox.cc:4386)
by ARDOUR::TriggerBoxThread::thread_work() (triggerbox.cc:4314)
by ARDOUR::TriggerBoxThread::_thread_work(void*) (triggerbox.cc:4285)
by fake_thread_start(void*) (pthread_utils.cc:101)
by start_thread (pthread_create.c:477)
by clone (clone.S:95)
```
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