fix adding or removing items to an open BMenu

The menu would try to relayout itself, but fail to get the supermenu
bounds as the looper for it was not locked in that case.

This fixes the crash with two downsides:
* The menu width isn't adjusted to match the parent menu (mostly visible
in BMenuFields)
* There is some flickering as the menu is updated

Fixes #9863 (Network prefs part).