[Reg 5.6->5.7] QML Engine crashes on exit with self JS reference

1   QV4::PersistentValueStorage::getEngine             qv4persistent.cpp     245  0x57d7cbdf 
2   QV4::WeakValue::engine                             qv4persistent_p.h     199  0x57bc136e 
3   QV4::WeakValue::free                               qv4persistent.cpp     408  0x57d7d18b 
4   QV4::WeakValue::clear                              qv4persistent_p.h     204  0x57bbcf1f 
5   QV4::QObjectWrapper::destroyObject                 qv4qobjectwrapper.cpp 989  0x57e18e18 
6   QV4::MemoryManager::sweep                          qv4mm.cpp             462  0x57bea257 
7   QV4::MemoryManager::~MemoryManager                 qv4mm.cpp             656  0x57be90be 
8   QV4::MemoryManager::`scalar deleting destructor'   Qt5Qmld                    0x57d7320f 
9   QV4::ExecutionEngine::~ExecutionEngine             qv4engine.cpp         459  0x57d5f67b 
10  QV4::ExecutionEngine::`scalar deleting destructor' Qt5Qmld                    0x57f1b5df 
11  QV8Engine::~QV8Engine                              qv8engine.cpp         170  0x57f19204 
12  QV8Engine::`vector deleting destructor'            Qt5Qmld                    0x57f1b50d 
13  QJSEngine::~QJSEngine                              qjsengine.cpp         315  0x57cd0111 
14  QQmlEngine::~QQmlEngine                            qqmlengine.cpp        1015 0x57e51358 
15  QQmlEngine::`scalar deleting destructor'           Qt5QuickWidgetsd           0x5956c5d0 
16  QObjectPrivate::deleteChildren                     qobject.cpp           1970 0x57501da9 
17  QWidget::~QWidget                                  qwidget.cpp           1696 0x582bda61 
18  QQuickWidget::~QQuickWidget                        qquickwidget.cpp      537  0x59563358 
19  QQuickWidget::`scalar deleting destructor'         selfcrash                  0x1362f70  
20  QObjectPrivate::deleteChildren                     qobject.cpp           1970 0x57501da9 
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