crash when new QMenuBar

I created one thread to run qml ApplicationWindow, then create QMainWindow in main thread, the app crash when new QMenubar in main thread.

I got that after debugging:
void QMenubarPrivate::init()

{ ...... HandleReparent();       // it result in crash ... extension = new ... ...... }

I found handlereparent will issue some events and then access 'extension' which will be created later.

I think maybe we should reconsider the order of the code lines.