fix: workaround a mypy error
fix: workaround a mypy error

This workaround is required, because mypy cannot distinguish
a Callable member attribute from a member function.
This workaround hides the member attribute between an Optional
and provides access to it through a property. The property
will assert if the Optional is None, making it non-Optional.

PolyJIT/benchbuild,PolyJIT/benchbuild,PolyJIT/benchbuild,PolyJIT/benchbuild