Okay I can recreate this on my Mac but I can see and report the details in the "Embedded Python Error"
Code:
Traceback (most recent call last):
File "/Volumes/SSD-Drive/repo/build2/bin/Sigil.app/Contents/python3lib/xmlprocessor.py", line 192, in repairXML
newdata = Opf_Parser(newdata).rebuild_opfxml()
File "/Volumes/SSD-Drive/repo/build2/bin/Sigil.app/Contents/python3lib/opf_newparser.py", line 422, in rebuild_opfxml
xmlres.append(self.convert_spine_attr_to_xml())
File "/Volumes/SSD-Drive/repo/build2/bin/Sigil.app/Contents/python3lib/opf_newparser.py", line 381, in convert_spine_attr_to_xml
for key in attr:
TypeError: 'NoneType' object is not iterable
The code that fails is trying to repair your opf as it is not well formed (every attribute in xml must be quoted) but gets confused trying to repair it causing the error.
Since Sigil requires a fully well-formed opf to function, even temporarily leaving the opf is a broken state is a serious issue for Sigil. Unfortunately, making a "robust" pure xml parser is hard without access to the exact schema used for that xml document. In the future I recommend using the MetaData Editor to make metadata changes as they will always be properly quoted preventing this error.