QMenu closes unexpectedly and action slot is not called

See attached example. A QMenu will be opened when the "MENU" button is pressed. In this menu the button "BUTTON" does nothing, except printing to stdout when pressed. Especially the menu stays open. The submenu below the button contains three entries. While this submenu is still open (i.e. mouse hovered over "SUBMENU") and the "BUTTON" is pressed at the same time, the complete menu closes and the button's slot is not called.