Plugin Error: Security accessibility of the overriding method must match the security accessibility of the method being overriden

   public void Execute(IServiceProvider serviceProvider)
        {
            ITracingService tracingService = (ITracingService)serviceProvider.GetService(typeof(ITracingService));
            IPluginExecutionContext executionContext = (IPluginExecutionContext)serviceProvider.GetService(typeof(IPluginExecutionContext));
            IOrganizationServiceFactory servicefactory = (IOrganizationServiceFactory)serviceProvider.GetService(typeof(IOrganizationServiceFactory));
            IOrganizationService service = servicefactory.CreateOrganizationService(executionContext.UserId);

            try
            {
                XrmServiceContext context = new XrmServiceContext(service);

                EntityReference leadid = (EntityReference)executionContext.InputParameters["LeadId"];

                #region Check Activities
                var activities = context.ActivityPointerSet.Where(m => m.RegardingObjectId == leadid.Id);

                if (activities == null || activities.Count() < 1)
                {
                    throw new InvalidPluginExecutionException("Lütfen Nitelikli hale getirmeden önce aktivite ekleyiniz.");
                }

                #endregion

                #region Check Fields
                Lead lead = (Lead)context.LeadSet.Where(m => m.Id == leadid.Id).FirstOrDefault();

                if (lead != null)
                {
                    if (lead.demo_jobtitle == null || lead.Telephone1 == null || lead.MobilePhone == null || lead.EMailAddress1 == null)
                    {
                        throw new InvalidPluginExecutionException("Unvan, telefon/cep telefonu ve email alanlarinin doldurulmasi zorunludur.");
                    }
                }
                #endregion
            }

            catch (FaultException<OrganizationServiceFault> ex)
            {
                throw new InvalidPluginExecutionException("An error occurred in the FollowupPlugin plug-in.", ex);
            }
        }