fails to build mononoki font from glyphs file (Loading Glyphs file failed: 'NoneType' object has no attribute 'groups')

fontmake currently fails to build the mononoki fonts from its glyphs sources (ufo sources succeed after a bug fix).

https://github.com/madmalik/mononoki.git

```sh
$ fontmake -o ttf -g src/mononoki.glyphs
INFO:fontmake.font_project:Building master UFOs and designspace from Glyphs source
INFO:glyphsLib.classes:Parsing "src/mononoki.glyphs" file into
fontmake: Error: In 'src/mononoki.glyphs': Loading Glyphs file failed: 'NoneType' object has no attribute 'groups'
```

Bug report: https://github.com/madmalik/mononoki/issues/99