Fix admin gig edit

Admin gig edit gets no gig data when the gigId is invalid. The page elements (except the title) therefore never loads.

Maybe show an error message or something?

Locate the admin gig edit entry point and reproduce the page with an invalid gigId. Check how the page handles missing gig data; done means the page responds visibly to the invalid id instead of leaving its elements unloaded, with the chosen behavior confirmed by an appropriate test or manual check.

索引モデルが issue の本文から書いたものです。