Crash due to QMultiInputContext creating widget while application is destroyed

Reason for the crash:

• The QApplication destructor sets allWidgets QSet to 0 before deleting all orphan widgets.
• During the widget deletion, qwidget_x11 QWidget::destroy calls inputContext() to get the current application's input context.
• If no input context has been created for the application yet, it creates an input context.
• QMultiInputContext creates a QMenu widget, which again is added to allWidgets QSet. Since this is already 0, this will
  crash.

Crash reported as KDE bugs 212618, 212857, 218878, 219973, 220853 (and possibly more duplicates).