Calva uses a text document for the repl output, which vscode requires to be stored on disk somewhere, so Calva puts it in a dot directory at the top of the workspace. No workspace, nowhere to put the file. The “cannot read 0” message, I believe, is it trying to select the folder from a list. There is no list, so it’s undefined. Choosing the first one from a non-existent list gives the error.
Opening the folder your file is in might work, that’ll set the directory as the vscode workspace. I haven’t tried it, though.