how to free menus memory

Wow, thanks - that's a bit worrying. I've just put a fix in for that issue with :undefined for the type of the menu. At least in my Bangle I don't see the memory increase though.

BTW what is the function arguments for in the function callback?

It's just to be able to access the underlying menu (the same thing that E.showMenu(mainmenu) returns). But maybe it's not needed - if we had some way of checking apps to ensure ther weren't using it we could remove it, and it'd make it far less likely memory leaked