[PyQt] Tracking down a random segfault inside QMetaObject::activate(QObject*, int, int, void**)

[Nikolaus Waxweiler]

Alright, "The-Compiler" from #pyqt helped me track down the offending
line. We were tracking references to the window in focus to work around
a bug we can't reproduce anymore. Since this code was also invoked after
a window closed, that window object might have been deleted already by
the time we checked the reference. We removed the code.