Found when package testing Qt 5.6.2.

When running the "Camera" example (using Qt Widgets) on Android, the viewfinder window is all black, and the following warning is visible on the console:

D Camera  : app passed NULL surface
W libcamera.so: (null):0 ((null)): The video surface is not compatible with any format supported by the camera

Also:

W Camera-JNI: callback on dead camera object

This is a Nexus 5 with Android 6.0.1.