--generate-cff option is broken

[khaledhosny]

$ compreffor LibertinusMath-Regular.hint.otf o.otf --generate-cff
Traceback (most recent call last):
  File "/env/bin/compreffor", line 10, in <module>
    sys.exit(main())
  File "/env/lib/python3.7/site-packages/compreffor/__main__.py", line 119, in main
    font['CFF '].cff.compile(f, None)
  File "/env/lib/python3.7/site-packages/fontTools/cffLib/__init__.py", line 105, in compile
    if otFont.recalcBBoxes and not isCFF2:
AttributeError: 'NoneType' object has no attribute 'recalcBBoxes'