[REG 5.5.1 -> 5.6.0] Planets example crashes on exit

1. Build and run the Planets example (Examples\Qt-5.6\qtcanvas3d\canvas3d\canvas3d\threejs\planets\planets.pro) on Qt 5.6.
2. Exit the example by closing its window.
   The example crashes in QListData::size where the d-pointer is invalid:

   1	QListData::size	qlist.h	107	0x5c44a9d1	
   2	QList<QtCanvas3D::CanvasTextureImage *>::indexOf	qlist.h	962	0x5b84f943	
   3	QList<QtCanvas3D::CanvasTextureImage *>::removeOne	qlist.h	896	0x5b850027	
   4	QtCanvas3D::CanvasTextureImageFactory::handleImageDestroyed	teximage3d.cpp	142	0x5b84cea6	
   5	QtCanvas3D::CanvasTextureImage::~CanvasTextureImage	teximage3d.cpp	203	0x5b84cf8a	
   6	QtCanvas3D::CanvasTextureImage::`scalar deleting destructor'	qtcanvas3dd		0x5b84ec0f	
   7	QV4::QObjectWrapper::destroyObject	qv4qobjectwrapper.cpp	1036	0x5c0444b9	
   8	QV4::MemoryManager::sweep	qv4mm.cpp	435	0x5be287e8	
   9	QV4::MemoryManager::~MemoryManager	qv4mm.cpp	625	0x5be2767e	
   10	QV4::MemoryManager::`scalar deleting destructor'	Qt5Qmld		0x5bfaeaff	
   11	QV4::ExecutionEngine::~ExecutionEngine	qv4engine.cpp	503	0x5bf997ac	
   12	QV4::ExecutionEngine::`scalar deleting destructor'	Qt5Qmld		0x5c148bef	
   13	QV8Engine::~QV8Engine	qv8engine.cpp	161	0x5c146613	
   14	QV8Engine::`vector deleting destructor'	Qt5Qmld		0x5c148b1d	
   15	QJSEngine::~QJSEngine	qjsengine.cpp	287	0x5bf0b305	
   16	QQmlEngine::~QQmlEngine	qqmlengine.cpp	971	0x5c08a60b	
   17	QQmlEngine::`scalar deleting destructor'	Qt5Quickd		0x5e0f3bd0	
   18	QObjectPrivate::deleteChildren	qobject.cpp	1963	0x5c739c29	
   19	QObject::~QObject	qobject.cpp	1036	0x5c7354ba	
   20	QWindow::~QWindow	qwindow.cpp	207	0x5d74a61c	
   ...	<Mehr>				
   

When you build the same example on Qt 5.5.1, it does not crash.