Unable to add a hosted feature service to the map viewer created from an empty feature class in a file geodatabase and edited once. The following error message is received: "The layer, [layer_name], is missing spatial reference information and cannot be added to the map."
Publish the empty feature class from ArcGIS for Desktop. Alternatively, add the file geodatabase as an item. Use 'Create layer' in ArcGIS Online and select the empty feature class to create the layer from (before any edits are performed).