Test and fix setting workflow to nothing

app/models/collection/workflowing.rb

@@ -13,6 +13,6 @@ module Collection::Workflowing
 
   private
     def set_all_cards_to_initial_workflow_stage
-      cards.update_all(stage_id: initial_workflow_stage.id)
+      cards.update_all(stage_id: initial_workflow_stage&.id)
     end
 end

test/models/collection/workflowing_test.rb

@@ -5,4 +5,9 @@ class Collection::WorkflowingTest < ActiveSupport::TestCase
     collections(:writebook).update! workflow: workflows(:on_call)
     assert_equal [ collections(:writebook).initial_workflow_stage ], collections(:writebook).cards.reload.collect(&:stage).uniq
   end
+
+  test "remove stage from cards if workflow is removed" do
+    collections(:writebook).update! workflow: nil
+    assert_equal [], collections(:writebook).cards.reload.collect(&:stage).compact
+  end
 end