public class AppEventReceiver : IRemoteEventService
{
    public SPRemoteEventResult ProcessEvent(SPRemoteEventProperties properties)
    {
        SPRemoteEventResult result = new SPRemoteEventResult();

        using (ClientContext clientContext = TokenHelper.CreateAppEventClientContext(properties, false))
        {
            if (clientContext != null)
            {
                clientContext.Load(clientContext.Web, ev=>ev.EventReceivers);
                clientContext.ExecuteQuery();

                // Create new List Added Event Receiver
                EventReceiverDefinitionCreationInformation listAddedEV = new EventReceiverDefinitionCreationInformation()
                {
                    EventType = EventReceiverType.ListAdded,
                    ReceiverName = "List_Events",
                    ReceiverUrl = GetServiceURL("List_Events.svc"),
                    SequenceNumber = 1000
                };

                // Add Event Receiver to Web's EventReceivers.
                EventReceiverDefinitionCollection webEvs = clientContext.Web.EventReceivers;

                List<EventReceiverDefinition> existingEvs = webEvs.Where(n => n.ReceiverName == "List_Events").ToList(); // Error is here!

                foreach (EventReceiverDefinition ev in existingEvs)
                {
                    ev.DeleteObject();
                }

                // Add event receiver to Projects List
                webEvs.Add(listAddedEV);
                clientContext.ExecuteQuery();
            }
        }

        return result;
    }

context.Credentials = new NetworkCredential("user", "password", "domain");

var existingEvs = webEvs.ToList().Where(n => n.ReceiverName == "List_Events").ToList();