Crash in QQQuickPaintedItem

In my app I use various QQuickPaintedItems and show them in a simple scene.

I get a crash; valgrind gives me the following backtrace:

==4377== Invalid read of size 8
==4377== at 0x542771D: QSGOpaqueTextureMaterial::compare(QSGMaterial const*) const (qsgtexturematerial.cpp:297)
==4377== by 0x540169D: QSGBatchRenderer::Renderer::prepareOpaqueBatches() (qsgbatchrenderer.cpp:1499)
==4377== by 0x5406518: QSGBatchRenderer::Renderer::render() (qsgbatchrenderer.cpp:2384)
==4377== by 0x541AF9F: QSGRenderer::renderScene(QSGBindable const&) (qsgrenderer.cpp:261)
==4377== by 0x541ADB1: QSGRenderer::renderScene() (qsgrenderer.cpp:220)
==4377== by 0x5434660: QSGRenderContext::renderNextFrame(QSGRenderer*, unsigned int) (qsgcontext.cpp:356)
==4377== by 0x548C3FC: QQuickWindowPrivate::renderSceneGraph(QSize const&) (qquickwindow.cpp:386)
==4377== by 0x545A6A3: QSGGuiThreadRenderLoop::renderWindow(QQuickWindow*) (qsgrenderloop.cpp:370)
==4377== by 0x545AD0D: QSGGuiThreadRenderLoop::event(QEvent*) (qsgrenderloop.cpp:466)
==4377== by 0x62EE503: QApplicationPrivate::notify_helper(QObject*, QEvent*) (qapplication.cpp:3501)
==4377== by 0x62EBC95: QApplication::notify(QObject*, QEvent*) (qapplication.cpp:2954)
==4377== by 0x7C131E3: QCoreApplication::notifyInternal(QObject*, QEvent*) (qcoreapplication.cpp:935)
==4377== Address 0x0 is not stack'd, malloc'd or (recently) free'd
==4377==
==4377==
==4377== Process terminating with default action of signal 11 (SIGSEGV)
==4377== Access not within mapped region at address 0x0
==4377== at 0x542771D: QSGOpaqueTextureMaterial::compare(QSGMaterial const*) const (qsgtexturematerial.cpp:297)
==4377== by 0x540169D: QSGBatchRenderer::Renderer::prepareOpaqueBatches() (qsgbatchrenderer.cpp:1499)
==4377== by 0x5406518: QSGBatchRenderer::Renderer::render() (qsgbatchrenderer.cpp:2384)
==4377== by 0x541AF9F: QSGRenderer::renderScene(QSGBindable const&) (qsgrenderer.cpp:261)
==4377== by 0x541ADB1: QSGRenderer::renderScene() (qsgrenderer.cpp:220)
==4377== by 0x5434660: QSGRenderContext::renderNextFrame(QSGRenderer*, unsigned int) (qsgcontext.cpp:356)
==4377== by 0x548C3FC: QQuickWindowPrivate::renderSceneGraph(QSize const&) (qquickwindow.cpp:386)
==4377== by 0x545A6A3: QSGGuiThreadRenderLoop::renderWindow(QQuickWindow*) (qsgrenderloop.cpp:370)
==4377== by 0x545AD0D: QSGGuiThreadRenderLoop::event(QEvent*) (qsgrenderloop.cpp:466)
==4377== by 0x62EE503: QApplicationPrivate::notify_helper(QObject*, QEvent*) (qapplication.cpp:3501)
==4377== by 0x62EBC95: QApplication::notify(QObject*, QEvent*) (qapplication.cpp:2954)
==4377== by 0x7C131E3: QCoreApplication::notifyInternal(QObject*, QEvent*) (qcoreapplication.cpp:935)