Unhandled exception in camera focus handling on Windows Phone

It's possible to get an unhandled exception when trying to focus with a lumia phone:

Steps to reproduce:

• Open declarative-camera example
• Switch to back camera
• "Focus" (causes 'ASSERT failure in bool __cdecl QWinRTCameraControl::focus(void): "The application called an interface that was marshalled for a different thread.", file D:\dev\source\qt5.8\qtmultimedia\src\plugins\winrt\qwinrtcameracontrol.cpp, line 1265')
• "Focus" button becomes "Focusing" (and does not return to "Focus" when focusing is finished)
• Press "Focusing" button -> Unhandled exception at 0x67B039E5 (winrtengined.dll) in declarative-camera.exe: 0xC0000005: Access violation reading location 0x00000000.