Summary: Smart editor widget requiring field when it's not a required field. Using WAB (online edition) with dashboard theme.
'Polygon Line Key' and 'Non-Territorial Line Key' are esriFieldTypeGUID, but they are not required (i.e. nullable: true). Smart editor widget requires these fields to be populated in order to save feature. These are the only two fields that show the message 'The value entered is not valid.' This message makes sense because I didn't enter a guid, but since it's a nullable field, these values should be allowed to be blank.
Creating features via the editor widget works fine as does creating features in a web map.