fix: workaround for loopback 2.x circular json error

lib/mixins/state-machine.js

@@ -156,6 +156,17 @@ module.exports = function StateMachine(Model, settings) {
       // Run the event method.
       return fms[eventName](this, ...args)
         .then(result => result)
+        .catch(err => {
+          if (Model.app.loopback.version.startsWith(2)) {
+            try {
+              JSON.stringify(err)
+            }
+            catch (e) {
+              _.unset(err, '[email protected]')
+            }
+          }
+          throw err
+        })
         .finally(() => Model.app.deleteStateMachine(this))
     }
   })