In Agent Flow, it seems that it's mandatory to SELECT a cluster when you use Spark SQL. It's NOT possible to test the query and see the preview without a cluster selected. If nothing is selected, a random error is generating showing there is a key issue. It would be good to make the cluster field mandatory as it seems it is mandatory to run a spark sql query.
When clicking on "Playground" this is the error message received if the cluster is not selected:
