Crash in app::Doc::needsBackup

[sentry]

Sentry Issue: ASEPRITE-H6

EXCEPTION_ACCESS_VIOLATION_READ / 0x188e65858c8: Fatal Error: EXCEPTION_ACCESS_VIOLATION_READ / 0x188e65858c8
  File "doc.cpp", line 307, in app::Doc::needsBackup
    return m_undo->canUndo() || m_undo->canRedo();
  File "backup_observer.cpp", line 179, in app::crash::BackupObserver::saveDocData
    if (!doc->needsBackup())
  File "backup_observer.cpp", line 156, in app::crash::BackupObserver::backgroundThread
    if (saveDocData(doc)) {
  File "backup_observer.cpp", line 65, in app::crash::BackupObserver::<lambda>::operator()
    , m_thread([this]{ backgroundThread(); })
  File "type_traits", line 1564, in std::invoke
    return static_cast<_Callable&&>(_Obj)();
...
(4 additional frame(s) were not displayed)

[martincapello]

I couldn't reproduce this crash. Nor was able to spot misbehaving code...At first I suspected that maybe the document was being deleted before the BackupObserver::backgroundThread calls BackupObserver::saveDocData, but when I followed the execution paths all seemed to be well protected by using mutexes...then I tried to see if there was any chance that m_undo was being set to null, but couldn't find anything either. So I gave up for now and move on to another issue.