Problem of the `ExposeThing` where the `formIndex` type is explicitly overridden

[danielpeintner]

Hmm, it seems to me as if this is a problem of the ExposeThing where the formIndex type is explicitly overridden to be non-nullable. Therefore, I would suggest keeping this as is for now and applying a more general fix as a follow-up.

Originally posted by @JKRhb in #994 (comment)