QCocoaApplication_mac crashes while QT running as a plug-in

QT crashes in the function below:

• (BOOL)qt_filterEvent:(NSEvent *)event
  {
  // crash if qApp is NULL
  if (qApp->macEventFilter(0, reinterpret_cast<EventRef>(event)))
  return true;

Because 'qApp' can be deleted while running QT as a plug-in, above lines should check 'qApp' before using it.

The function is invoked by 'qt_sendEvent_replacment' which is a reimplemented method of 'sendEvent' of NSApplication. It is always existing even Qt instance has been killed.

It is necessary to recover original methods when the Qt instance is destructed.