AEM 6.5.11 workflow participant step can't send email.
Hi guys,
My AEM version is 6.5.11.
I created a workflow only contains participant step, selected a group and checked the "Notify user via email."


when I started the workflow, it will throw error and email sent fail:
*ERROR* [FelixDispatchQueue] org.apache.felix.eventadmin FrameworkEvent ERROR (org.osgi.framework.ServiceException: Service factory returned null. (Component: com.day.cq.workflow.impl.email.EMailNotificationService (3295)))
org.osgi.framework.ServiceException: Service factory returned null. (Component: com.day.cq.workflow.impl.email.EMailNotificationService (3295))
at org.apache.felix.framework.ServiceRegistrationImpl.getFactoryUnchecked(ServiceRegistrationImpl.java:381)
at org.apache.felix.framework.ServiceRegistrationImpl.getService(ServiceRegistrationImpl.java:248)
at org.apache.felix.framework.ServiceRegistry.getService(ServiceRegistry.java:350)
at org.apache.felix.framework.Felix.getService(Felix.java:3954)
at org.apache.felix.framework.BundleContextImpl.getService(BundleContextImpl.java:450)
at org.apache.felix.eventadmin.impl.handler.EventHandlerProxy.obtain(EventHandlerProxy.java:286)
at org.apache.felix.eventadmin.impl.handler.EventHandlerProxy.sendEvent(EventHandlerProxy.java:407)
at org.apache.felix.eventadmin.impl.tasks.HandlerTask.runWithoutBlacklistTiming(HandlerTask.java:82)
at org.apache.felix.eventadmin.impl.tasks.SyncDeliverTasks.execute(SyncDeliverTasks.java:104)
at org.apache.felix.eventadmin.impl.handler.EventAdminImpl.sendEvent(EventAdminImpl.java:147)
at org.apache.felix.eventadmin.impl.security.EventAdminSecurityDecorator.sendEvent(EventAdminSecurityDecorator.java:96)
at com.day.cq.workflow.compatibility.CQEventDispatcher.handleEvent(CQEventDispatcher.java:83)
at org.apache.felix.eventadmin.impl.handler.EventHandlerProxy.sendEvent(EventHandlerProxy.java:415)
at org.apache.felix.eventadmin.impl.tasks.HandlerTask.runWithoutBlacklistTiming(HandlerTask.java:82)
at org.apache.felix.eventadmin.impl.tasks.SyncDeliverTasks.execute(SyncDeliverTasks.java:104)
at org.apache.felix.eventadmin.impl.tasks.AsyncDeliverTasks$TaskExecuter.run(AsyncDeliverTasks.java:166)
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
at java.util.concurrent.FutureTask.run(FutureTask.java:266)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
at java.lang.Thread.run(Thread.java:748)
*ERROR* [EventAdminAsyncThread #2] com.day.cq.workflow.cq-workflow-impl bundle com.day.cq.workflow.cq-workflow-impl:6.3.8.CQ652-B0003 (486)[com.day.cq.workflow.impl.email.EMailNotificationService(3295)] : The activate method has thrown an exception (java.lang.ClassCastException: java.lang.String cannot be cast to java.lang.Boolean)
java.lang.ClassCastException: java.lang.String cannot be cast to java.lang.Boolean
at com.day.cq.workflow.impl.email.EMailNotificationService.activate(EMailNotificationService.java:447)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at org.apache.felix.scr.impl.inject.methods.BaseMethod.invokeMethod(BaseMethod.java:242)
at org.apache.felix.scr.impl.inject.methods.BaseMethod.access$500(BaseMethod.java:41)
at org.apache.felix.scr.impl.inject.methods.BaseMethod$Resolved.invoke(BaseMethod.java:678)
at org.apache.felix.scr.impl.inject.methods.BaseMethod.invoke(BaseMethod.java:524)
at org.apache.felix.scr.impl.inject.methods.ActivateMethod.invoke(ActivateMethod.java:318)
at org.apache.felix.scr.impl.inject.methods.ActivateMethod.invoke(ActivateMethod.java:308)
at org.apache.felix.scr.impl.manager.SingleComponentManager.createImplementationObject(SingleComponentManager.java:342)
at org.apache.felix.scr.impl.manager.SingleComponentManager.createComponent(SingleComponentManager.java:115)
at org.apache.felix.scr.impl.manager.SingleComponentManager.getService(SingleComponentManager.java:984)
at org.apache.felix.scr.impl.manager.SingleComponentManager.getServiceInternal(SingleComponentManager.java:957)
at org.apache.felix.scr.impl.manager.SingleComponentManager.getService(SingleComponentManager.java:902)
at org.apache.felix.framework.ServiceRegistrationImpl.getFactoryUnchecked(ServiceRegistrationImpl.java:348)
at org.apache.felix.framework.ServiceRegistrationImpl.getService(ServiceRegistrationImpl.java:248)
at org.apache.felix.framework.ServiceRegistry.getService(ServiceRegistry.java:350)
at org.apache.felix.framework.Felix.getService(Felix.java:3954)
at org.apache.felix.framework.BundleContextImpl.getService(BundleContextImpl.java:450)
at org.apache.felix.eventadmin.impl.handler.EventHandlerProxy.obtain(EventHandlerProxy.java:286)
at org.apache.felix.eventadmin.impl.handler.EventHandlerProxy.sendEvent(EventHandlerProxy.java:407)
at org.apache.felix.eventadmin.impl.tasks.HandlerTask.runWithoutBlacklistTiming(HandlerTask.java:82)
at org.apache.felix.eventadmin.impl.tasks.SyncDeliverTasks.execute(SyncDeliverTasks.java:104)
at org.apache.felix.eventadmin.impl.handler.EventAdminImpl.sendEvent(EventAdminImpl.java:147)
at org.apache.felix.eventadmin.impl.security.EventAdminSecurityDecorator.sendEvent(EventAdminSecurityDecorator.java:96)
at com.day.cq.workflow.compatibility.CQEventDispatcher.handleEvent(CQEventDispatcher.java:83)
Can anyone help me?
Thank you in advance!