A now deleted comment suggested I add line 77 there to deal with the magic strings (both this and the one in the other comment). I am a bit uncertain of how else to avoid the strings but since line 77 can only trigger if someone added a new mode with an unsupported antag type or an admin somehow var edited a mode's secondary antag before roundstart (not sure how they'd even do that) I think I can reasonably say it's a safe magic string, even if it is a bit ugly.

The way the code runs that forced error is the only new code an invalid secondary_antag would actually run past the "choose a secondary antag" code. You just wouldn't get (a) secondary antag(s).