Fix crash regression for ApplicationWindow on Mac

This fixes a recent regression following the integration of
change:

7cc1656fef21e6bdc044968a79f0a41155357c29
(Make sure menu bar has a parent window ...)

As it now consistently crashes on mac in several of the
Qt Quick Controls examples, it is critical that it gets into
the release branch.

Change-Id: If3db1025229a7f7fd4e7ecc703d5f655db73964d
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>

src/plugins/platforms/cocoa/qcocoamenubar.mm

@@ -175,6 +175,7 @@ void QCocoaMenuBar::handleReparent(QWindow *newParentWindow)
     if (newParentWindow == NULL) {
         m_window = NULL;
     } else {
+        newParentWindow->create();
         m_window = static_cast<QCocoaWindow*>(newParentWindow->handle());
         m_window->setMenubar(this);
     }