Webforms downloading results data is broken

when attempting to download webform responses as a .xlsx or .csv file the system appears to build a data set but no data is in the resulting file downloaded by the browser.

There are many Notices in the log:

Notice: Trying to access array offset on value of type null in _webform_csv_data_select() (line 855 of /code/sites/all/modules/contrib/webform/components/select.inc).

These notices may or may not be relevant.