code: void setActiveSubMenu (PopupMenu newSubMenu) {
  		if (activeSubMenu == newSubMenu) return;
  		if (activeSubMenu != null) activeSubMenu.remove();
  		activeSubMenu = newSubMenu;
  		if (newSubMenu != null) {
  			newSubMenu.setParentMenu(this);
  		}
  	}
docs: Called by framework, when PopupMenu is added to MenuItem as submenu