System.NullReferenceException: Object reference not set to an instance of an object.
at Nd.Platform.MongoAccess.MongoSession.Get[T](ShardParams shardParams, Object id)
at Nd.Platform.MongoAccess.MongoShardSession`1.Get(Object id)
at Castle.Proxies.TrainingSerialRepositoryProxy.Get_callback_1(ShardParams shardParams, Object id)
at Castle.Proxies.Invocations.AbstractRepository`1_Get_24.InvokeMethodOnTarget()
at Castle.DynamicProxy.AbstractInvocation.Proceed()
at Nd.Platform.RepositoryGetMethodInvocation.Process(IInvocation invocation, ClassDefineMetadata metadata)
at Nd.Platform.RepositoryInterceptor.Intercept(IInvocation invocation)
at Castle.DynamicProxy.AbstractInvocation.Proceed()
at Castle.Proxies.TrainingSerialRepositoryProxy.Get(ShardParams shardParams, Object id)
at Nd.Repository.Practice.TrainingSerialRepository.Get(String serialId)
at Nd.Platform.Practice.TrainingSerialService.UpdateSerial(String serialId, UserQuestionResult userAnswer, Int64 clientId)
at Nd.Platform.Web.Areas.ThirdApp.Controllers.ChapterController.SetCellsForAppLearning(Int32 appId, Int32 unitId, String serialCode, UserQuestionResult userAnswer)
at lambda_method(Closure , ControllerBase , Object[] )
at System.Web.Mvc.ReflectedActionDescriptor.Execute(ControllerContext controllerContext, IDictionary`2 parameters)
at System.Web.Mvc.ControllerActionInvoker.InvokeActionMethod(ControllerContext controllerContext, ActionDescriptor actionDescriptor, IDictionary`2 parameters)
at Nd.Web.Mvc.ApiControllerActionInvoker.InvokeActionMethod(ControllerContext controllerContext, ActionDescriptor actionDescriptor, IDictionary`2 parameters)
at System.Web.Mvc.ControllerActionInvoker.c__DisplayClass15.b__12()
at System.Web.Mvc.ControllerActionInvoker.InvokeActionMethodFilter(IActionFilter filter, ActionExecutingContext preContext, Func`1 continuation)
at System.Web.Mvc.ControllerActionInvoker.InvokeActionMethodFilter(IActionFilter filter, ActionExecutingContext preContext, Func`1 continuation)
at System.Web.Mvc.ControllerActionInvoker.InvokeActionMethodWithFilters(ControllerContext controllerContext, IList`1 filters, ActionDescriptor actionDescriptor, IDictionary`2 parameters)
at System.Web.Mvc.ControllerActionInvoker.InvokeAction(ControllerContext controllerContext, String actionName)