I thought this may help some people that upgrade to 3.7.18
Even with the fix from juggledad regarding the functions file, eg changes to line 925
if (is_single()) {
I still had the issue of categories showing on my menu 2. Having saved the various page settings, eg content, header and footer they were still showing.
However i noticed when I went into the Menu's option, under Theme locations, my 2nd menu was not ticked:
Theme location> Menu 2
ticking this, solved the issue and my menu returned to normal.
thought this would help people, who had tried the replacement function code and still had the issue.