Exception `Invalid_argument("Opt.get")` raised when extracting to C
This seems related to non-modular extraction:
> echo "let bar () = ()" > foo.mlw
> bin/why3 extract --debug=stack_trace -Dc foo.mlw
Fatal error: exception Invalid_argument("Opt.get")
Raised at file "stdlib.ml", line 34, characters 20-45
Called from file "src/util/opt.ml" (inlined), line 16, characters 27-48
Called from file "src/extract/c.ml", line 557, characters 48-72
Called from file "src/extract/c.ml", line 819, characters 6-28