Error: argument of the type 'NoneType' is not iterable

Please help me resolve this error.
i am encountering this error upon previewing the form. the form was generated as xlsform and got uploaded successfully.

Hi this is most likely related to an ongoing bug KoBo is working to resolve - https://github.com/kobotoolbox/kpi/issues/2175

It’s most likely caused by an unlabeled language column.