retrieve unicode on error --> bug as in py3

[typemytype]

traceback :

I've been receiving the following error when generating a new font when using Scalefast:

"File "/Applications/RoboFont.app/Contents/Resources/lib/python3.7/vanilla/vanillaBase.py", line 503, in action_
self.callback(sender)
File "scaleFast.py", line 533, in _generationCallback
File "scaleFast.py", line 726, in generateGlyphsToFont
File "scaleFast.py", line 756, in _buildScaledGlyphs
File "scaleFast.py", line 763, in _retrieveScaledGlyph
File "/Users/graemeswank/Library/Application Support/RoboFont/plugins/ScaleFast.roboFontExt/lib/mutatorScale/lib/mutatorScale/objects/scaler.py", line 266, in getScaledGlyph
instanceGlyph.unicodes = masters[0][glyphName].unicodes
TypeError: 'dict_values' object is not subscriptable"

see https://forum.robofont.com/topic/970/repeated-error-using-scalefast