[OS X] crash on exit from fullscreen mode (stale instance with this==NULL accessed in the Cocoa QPA)

A crash occurs when returning from fullscreen mode on OS X, in kdenlive5, while playing a clip.

1) start kdenlive 16.04.1 (the KF5 version)
2) add a clip to a new project
3) start the clip
4) double-click the clip to switch to fullscreen mode (or use the corresponding menu or toolbar command)
5) hit Esc to exit from fullscreen

The backtrace suggests that the Cocoa QPA calls a method of a deleted class (QCocoaWindow?) instance (this==NULL)

This crash does not occur when using the xcb QPA plugin (sic, on OS X).