Mac OS: In Qt Designer, cannot add QMenuItem objects to QMenu

Am following the Getting Started tutorial and am following instructions to add a Menu Item.

The "Type Here" entry is visible and I can enter my own text e.g. "File".

But after having created a QMenu entry, there is no way to open it to add QMenuItem objects to it... it remains closed regardless of click, double-click, long-click, right-click, Enter key press, cursor key presses.

QObject, QWidget and QMenu properties are all at default - see attached screenshot.