Picard crashes for some files if using scan function. I managed to reproduce the crash. The crash actually happens, when the loading of the album details are done and it is about to display the structure on the tree.

The log message is always the same, independently from the file:
E: 23:34:04,301 C:\/Users/user/AppData/Local/MusicBrainz/Picard/plugins/classical_extras.zip/classical_extras.write_log:256: Classical Extras: Found parent which is descendant of child - not using, to prevent circular references. id = d2904a5a-68a9-4b80-85cb-2ad1c75c52fb, name = String Quartet, op. 11
E: 23:34:04,302 C:\/Users/user/AppData/Local/MusicBrainz/Picard/plugins/classical_extras.zip/classical_extras.write_log:256: Classical Extras: Found parent which is descendant of child - not using, to prevent circular references. id = d2904a5a-68a9-4b80-85cb-2ad1c75c52fb, name = String Quartet, op. 11

I currently not working with classical music, but more contemporary kind. I don't know where the "String Quartet, op. 11" comes from.

It appears, that the call https://musicbrainz.org/ws/2/work/069a9da6-db37-4d1b-a85a-282cee4f4138?inc=work-rels+artist-rels+label-rels+place-rels+aliases+tags returns an invalid structure. Interestingly, all the crashes I encounter for the different files try to load this entry. 

Sadly, from the debug print I couldn't figure which file was (erroneously) fingerprinted as this Barber String Quartet, but if needed I can find and provide the culprit file.