Crash in QCocoaBackingStore::paintDevice()

My app uses Qt 5.3.1 on Mac OS X 10.9.2 (13C1021).

Occasionally it crashes within the Qt libraries, always at the same place. I've attached the diagnostic info from two such crashes, which contains full stack traces.

The callstack for the crash shows its trying to dereference a NULL pointer:
Thread 0 Crashed:: Dispatch queue: com.apple.main-thread
0 libqcocoa.dylib 0x0000000103be6520 QCocoaBackingStore::paintDevice() + 32
1 QtWidgets 0x00000001007cd4a4 QWidgetBackingStore::doSync() + 2964
2 QtWidgets 0x00000001007cd7ee QWidgetBackingStore::sync() + 494
3 QtWidgets 0x00000001007eca03 QWidgetPrivate::syncBackingStore() + 211
4 QtWidgets 0x00000001007fa96d QWidget::event(QEvent*) + 2637
5 QtWidgets 0x00000001008f07e7 QMainWindow::event(QEvent*) + 935