Excuse me for asking, but why do you want an extra button for this?
If you used a keybinding to open the map you can just press it again (or press esc) and if you're a clicker you can just click the close button?
Nevertheless I quickly tested it on beta and
securecall("CloseAllWindows") (called by
ToggleGameMenu()) still returns the WorldMapFrame after hiding the frame, that's why it stops there and doesnt open the menu.
Just hiding the map frame isn't the same as closing it - open your character frame after hiding the map, it will not be correctly anchored to the left side of the screen, instead it's next to the invisible map and fixing that requires calling secure code which we can't do.