Thank you for the additional information.
When RadGrid is used as FilterContainer, filtering can be applied only on the data fields bound to the grid. RadFilter itself has no reference to the DataSource of the grid, so using FilterEditor with FieldName that appears in the RadGrid's data source but is not bound to the grid itself, will cause null reference exception.
Filtering on a column bound to the RadGrid with Visibile property set to "false" is a working solution.
If you prefer to filter on DataSource control instead of the Grid, you can check out the Integration with DataSource Controls article.
Progress is here for your business, like always. Read more about the measures we are taking to ensure business continuity and help fight the COVID-19 pandemic.
Our thoughts here at Progress are with those affected by the outbreak.