FATAL error in Drupal\field\Plugin\views\field\Field::groupByForm

Steps to reproduce:

- Create a view with field rows, not entity
- Add atleast one field
- Enable 'Use aggregation' setting for the view under advanced fieldset
- click 'Aggregation settings' link for your field
- FATAL

The form is still trying to use field_info['columns'], assuming this is an array. This is a Field config entity now, but also doesn't have a columns property. So we can use the getColumns() method instead.

Here is the fix, don't have time to do test coverage right now, but will do later.