REST module returns error when trying to Publish a GET Service.

I have created a Publishing REST Service like I said in https://forum.mendixcloud.com/link/questions/86790

Since POST doesn't seem to work, I tried GET, hoping it would be easier, but this returns me an error as well:

Failed to serve GET testing/1: Exception occurred in action '{}', all database changes executed by this action were rolled back

--------

com.mendix.core.CoreRuntimeException: Exception occurred in action '{}', all database changes executed by this action were rolled back
	at com.mendix.core.actionmanagement.CoreAction.processErrorState(CoreAction.java:303)

Caused by: com.mendix.core.CoreException: java.lang.IllegalArgumentException: Super class should not be null
	at com.mendix.core.component.InternalCore.execute(InternalCore.java:422)

Caused by: java.lang.IllegalArgumentException: Super class should not be null
	at com.mendix.core.component.InternalCore.isSubClassOf(InternalCore.java:837)
	at com.mendix.modules.microflowengine.internal.MicroflowEngineModuleImpl$$anonfun$com$mendix$modules$microflowengine$internal$MicroflowEngineModuleImpl$$addParameters$2$1$$anonfun$apply$1.apply(MicroflowEngineModuleImpl.scala:90)
	at com.mendix.modules.microflowengine.internal.MicroflowEngineModuleImpl$$anonfun$com$mendix$modules$microflowengine$internal$MicroflowEngineModuleImpl$$addParameters$2$1$$anonfun$apply$1.apply(MicroflowEngineModuleImpl.scala:88)
	at scala.collection.Iterator$class.foreach(Iterator.scala:742)
	at scala.collection.AbstractIterator.foreach(Iterator.scala:1194)
	at scala.collection.IterableLike$class.foreach(IterableLike.scala:72)
	at scala.collection.AbstractIterable.foreach(Iterable.scala:54)
	at com.mendix.modules.microflowengine.internal.MicroflowEngineModuleImpl$$anonfun$com$mendix$modules$microflowengine$internal$MicroflowEngineModuleImpl$$addParameters$2$1.apply(MicroflowEngineModuleImpl.scala:88)
	at com.mendix.modules.microflowengine.internal.MicroflowEngineModuleImpl$$anonfun$com$mendix$modules$microflowengine$internal$MicroflowEngineModuleImpl$$addParameters$2$1.apply(MicroflowEngineModuleImpl.scala:73)
	at scala.collection.IndexedSeqOptimized$class.foreach(IndexedSeqOptimized.scala:33)
	at scala.collection.mutable.ArrayOps$ofRef.foreach(ArrayOps.scala:186)
	at com.mendix.modules.microflowengine.internal.MicroflowEngineModuleImpl.com$mendix$modules$microflowengine$internal$MicroflowEngineModuleImpl$$addParameters$2(MicroflowEngineModuleImpl.scala:73)
	at com.mendix.modules.microflowengine.internal.MicroflowEngineModuleImpl$$anonfun$createMicroflow$3.apply(MicroflowEngineModuleImpl.scala:108)
	at com.mendix.modules.microflowengine.internal.MicroflowEngineModuleImpl$$anonfun$createMicroflow$3.apply(MicroflowEngineModuleImpl.scala:108)
	at scala.Option.map(Option.scala:146)
	at com.mendix.modules.microflowengine.internal.MicroflowEngineModuleImpl.createMicroflow(MicroflowEngineModuleImpl.scala:108)
	at com.mendix.modules.microflowengine.internal.MicroflowEngineModuleImpl.createMicroflow(MicroflowEngineModuleImpl.scala:17)
	at com.mendix.core.action.ActionFactory.createUserAction(ActionFactory.scala:42)
	at com.mendix.core.component.InternalCore.execute(InternalCore.java:413)
	at com.mendix.core.Core.execute(Core.java:165)
	at restservices.publish.DataService.convertSourceToView(DataService.java:453)
	at restservices.publish.DataService.serializeToJson(DataService.java:460)
	at restservices.publish.DataService.serveGetFromDB(DataService.java:261)
	at restservices.publish.DataService.serveGet(DataService.java:236)
	at restservices.publish.DataService$4.execute(DataService.java:571)
	at restservices.publish.RestServiceHandler$3.apply(RestServiceHandler.java:225)
	at restservices.publish.RestServiceHandler$3.apply(RestServiceHandler.java:1)
	at restservices.util.Utils$1.executeAction(Utils.java:209)
	at com.mendix.systemwideinterfaces.core.UserAction.execute(UserAction.java:49)
	at com.mendix.core.actionmanagement.CoreAction.doCall(CoreAction.java:260)
	at com.mendix.core.actionmanagement.CoreAction.call(CoreAction.java:248)
	at com.mendix.core.actionmanagement.CustomActionMonitorImpl$$anonfun$run$1.apply(CustomMonitoredAction.scala:14)
	at com.mendix.core.session.Worker$$anonfun$receive$3$$anonfun$2$$anon$1.execute(ActionDispatching.scala:143)
	at com.mendix.util.classloading.Runner.doRunUsingClassLoaderOf(Runner.java:36)
	at com.mendix.core.session.Worker$$anonfun$receive$3$$anonfun$2.apply(ActionDispatching.scala:145)
	at scala.util.Try$.apply(Try.scala:192)
	at com.mendix.core.session.Worker$$anonfun$receive$3.applyOrElse(ActionDispatching.scala:139)
	at akka.actor.Actor$class.aroundReceive(Actor.scala:465)
	at com.mendix.core.session.Worker.aroundReceive(ActionDispatching.scala:135)
	at akka.actor.ActorCell.receiveMessage(ActorCell.scala:516)
	at akka.actor.ActorCell.invoke(ActorCell.scala:487)
	at akka.dispatch.Mailbox.processMailbox(Mailbox.scala:238)
	at akka.dispatch.Mailbox.run(Mailbox.scala:220)
	at akka.dispatch.ForkJoinExecutorConfigurator$AkkaForkJoinTask.exec(AbstractDispatcher.scala:393)
	at scala.concurrent.forkjoin.ForkJoinTask.doExec(ForkJoinTask.java:260)
	at scala.concurrent.forkjoin.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:1339)
	at scala.concurrent.forkjoin.ForkJoinPool.runWorker(ForkJoinPool.java:1979)
	at scala.concurrent.forkjoin.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:107)