When a subscribed filter gets deleted, there will be NullPointerExceptions when trying to send the results of the (non-existant) search request.

Need to create a consistency check to remove these subscriptions.

For the time being the SQL to find these subscriptions is:
SELECT * FROM filtersubscription where filter_i_d is not in (select id from searchrequest);

(Note, the above SQL will not work on MySQL due to a sub-select).