If you use the Primary Sources statement list dialog and click on "Approve" for one of the items, the progress bar spins forever and the console logs the error above.

The offending line is here:
https://github.com/google/primarysources/blob/master/frontend/freebase2wikidata.js#L1921

The statement gets added, but because the exception happens before the call to setStatementState, I suspect that the statement will get presented again, resulting in duplicates.