Summary:
It's not allowed to return nullptr from the callback. The assert ensures it which is helpful during development.
Probably, we should consider using `gsl::not_null<>` here.
Changelog: [Internal] Fabric-specific internal change.
Reviewed By: sammy-SC
Differential Revision: D21149891
fbshipit-source-id: a5f77b35029f22b499491721036405682f812a38