remove transition, do not destroy output_places

app/models/workflow_core/transition.rb

@@ -10,7 +10,7 @@ module WorkflowCore
 
     has_many :input_places, dependent: :nullify,
              foreign_key: "output_transition_id", class_name: "WorkflowCore::Place"
-    has_many :output_places, dependent: :destroy,
+    has_many :output_places, dependent: :nullify,
              foreign_key: "input_transition_id", class_name: "WorkflowCore::Place"
   end
 end