I investigated this for a short bit. It is only reproducible on Windows because here Qt will use the native file dialogs and when you make multiple selections in the native window Qt returns '' for the output path. There doesn't appear to be a way to prevent multi-selection in the native dialog using the Qt QFileDialog.getExistingDirectory() API.
If I add the QFileDialog::DontUseNativeDialog, the Qt dialog does not allow multi-selection and thus has no problems. See also:
This is still present in Workbench in Tortoise 2.0.
It still allows you to select multiple folders but it silently fails when you try to add them. For my two cents this would be extremely useful when you need to add several repositories to the Workbench.