What version is that? An earlier version (maybe... 0.4.6 and earlier?) had a bug in the UI where sometimes the config file would get set to 'undefined'. You can usually fix it by deleting 'config.json'. This bug has been fixed in the most recent version, 0.4.8
looks like Sia UI version: 0.4.6-beta Sia version: 0.4.4 (
https://github.com/NebulousLabs/Sia-UI/releases/tag/v0.4.6-beta , your link from the OP)
Downloaded the wallet the same day I published the error.
So I deleted the config.json and opened the wallet, it seems to be locked (?!), whats the password ?? it's empty anyway.. so I tried to load the legacy wallet and still getting this error

Same error happens on 0.4.8

strange : I have done successfully the following :
- download the new UI, synchronise
- create a new wallet with a password generated (of course you need to note this one)- upload legacy wallet (if asked, give the previous password, even if you legacy wallet is not password protected).
- done : after a few seconds, the balance of the legacy wallet appears.
Remark : you need to use the latest wallet.dat of you legacy wallet (previous saved ones may not include latest adresses used by latest transaction : this is the same for bitcoin).