Fix bug arising from PyQt4 where QtGui.qApp is not initialized to None

[quornian]

For example, in this first case we have what looks like a QApplication
object, but it is not really there and will cause problems later.

$ python -c 'from PyQt4 import QtGui; print QtGui.qApp'
<PyQt4.QtGui.QApplication object at 0x7f2f884776b0>

$ python -c 'from PySide import QtGui; print QtGui.qApp'
None

Instead we can use QtGui.QApplication.instance(), which always works and
is documented in PySide to return the same as QtGui.qApp anyway.

[quornian]

Hi @manneohrstrom. Is there any chance this can be merged?

To add more context, the code added by the two commits ac2ef1b and ca9059d makes use of QGui.qApp, which is not set to None in PyQt4 at startup (it's some uninitialized QApplication struct). In PySide it all works fine. Using QtGui.QApplication.instance() on the other hand, always works.

The problem apparently exists in PyQt5 too (with a much better explanation):
https://stackoverflow.com/a/40405396