I have just fixed this in my wip version

However, the reason for this error to occur (as far as I can tell anyway) is bad configuration...namely if Mamewah cannot find any 'emulators' set up. You need to create subfolders within \mamewah\config for each platform, and have the associated ini files within...eg:
\mamewah\config\mame\mame.ini
\mamewah\config\mame\mame-0.ini
\mamewah\config\mame\mame-1.ini
etc.
HTH