The Sparkle documentation mentions: “Popups can’t be nested, so one popup can’t open a different one.” But there is a way to do so, though it is a bit tricky: By placing an additional menu into the popup (and reducing the menu items to just one). Yes, the formatting options for menues seem to be reduced compared to “normal” text. It is possible to place a couple of those additional menus in a popup. Menu items placed into a popup are allowed to open (new) popups.
My suggestion: to make it easier to nest popups – to be able to use different popups for different submenus while having one popup as the main menu.