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
