multiselect: put back custom-label & fix

[icebob]

I removed custom-label property from multiselect because give errors.
If it props doesn't exist in schema, multiselect try to call and gives error.

[lionel-bijaoui]

I had the same problem while working the v2, and found a solution. It need to at least (even when empty) return a function. I was returning null and it was not happy.
I will update my master and do a PR with the fix.

[icebob]

Merged. Thanks!