OK, I activated plugin again and it didn't mess up sidebars. Then I tested adding more Nextend menus one main page at a time. Fine for about 3 tries, then 4th made sidebars disappear again. So, it (the problem) appears random. I deactivated plugin and sidebars are back.
I think maybe the solution is if I make any significant changes, I should deactivate plugin first, then make changes, then activate again.
OR, just deactivate and activate again when it happens, although this leaves site with no sidebar temporarily.
Of course, all this could react differently with different options checked in plugin's settings, yeah?
Anyway, thanks again Frank!