Mac: Menu bars unconditionally shown when setNativeMenuBar is called

We've got a problem in Qt Creator, that, when e.g. opening the preferences, a little toplevel window appears and vanishes again.
I tracked the problem down to

• Designer initializes and creates an instance of a QMenuBar subclass
• in the class' constructor, setNativeMenuBar(false) is called
• the implementation of that unconditionally calls "setVisible(false); setVisible(true);"
  ---> since the instance doesn't have a parent (yet) it gets shown as a toplevel

The setVisible calls are definitely wrong the way they are.
They solve the problem of setNativeMenuBar(false) being called while the menu is already shown as a native menu bar. (The other way round also works without the setVisible calls.)
See attached sample project (click the button to toggle the nativeMenuBar property).