You might want to consider to not allow adding LNbits demo mints at all to the list. It's unclear whether these will be there permanently.
Not sure the best way to fix it if it's an lnbits mint, this info endpoint should return a public key for the mint.
https://obsessedjerky7.lnbits.com/cashu/api/v1/L4bNfiBtTyhEpZWdt6QTb4/v1/info
We still add the mint, but just point it out to try to get you to follow the spec.
Discussion
Looks like just self-hosted lnbits so far, but good point