The issue was that the author moved the MaterialUI folder inside another folder. And here's the fix:
Try turning this line (just above that 'if(Directory.Exists..." line):
string[] materialUiDirectories = Directory.GetDirectories(Application.dataPath, "MaterialUI");
into this:
string[] materialUiDirectories = Directory.GetDirectories(Application.dataPath, "MaterialUI", SearchOption.AllDirectories);
and let me know if that helps :)
~ Declan.