I get this error message when I try to open some .otf files in Glyphs 3:
Error reading the document.
(-[__NSCFString appendString:]: nil argument)
The problem does not occur in Glyphs 2.
I think something needs to be changed in the original Glyphs file.
Because exporting a new .otf with Glyphs 3 doesn’t help and some other files are fine.