Set Lookup value into Null

// <copyright file="ChangeEndCustomer.cs" company="Microsoft">
// Copyright (c) 2017 All Rights Reserved
// </copyright>
// <author>Microsoft</author>
// <date>11/15/2017 4:34:10 PM</date>
// <summary>Implements the ChangeEndCustomer Workflow Activity.</summary>
namespace MFS_SOLUTION.MFS_WORKFLOW
{
    using System;
    using System.Activities;
    using System.ServiceModel;
    using Microsoft.Xrm.Sdk;
    using Microsoft.Xrm.Sdk.Workflow;
    using Microsoft.Xrm.Sdk.Messages;
    using Microsoft.Xrm.Sdk.Query;
    using System.Linq;

    public sealed class ChangeEndCustomer : CodeActivity
    {
        /// <summary>
        /// Executes the workflow activity.
        /// </summary>
        /// <param name="executionContext">The execution context.</param>
        protected override void Execute(CodeActivityContext executionContext)
        {
            // Create the tracing service
            ITracingService tracingService = executionContext.GetExtension<ITracingService>();

            if (tracingService == null)
            {
                throw new InvalidPluginExecutionException("Failed to retrieve tracing service.");
            }

            tracingService.Trace("Entered ChangeEndCustomer.Execute(), Activity Instance Id: {0}, Workflow Instance Id: {1}",
                executionContext.ActivityInstanceId,
                executionContext.WorkflowInstanceId);

            // Create the context
            IWorkflowContext context = executionContext.GetExtension<IWorkflowContext>();

            if (context == null)
            {
                throw new InvalidPluginExecutionException("Failed to retrieve workflow context.");
            }

            tracingService.Trace("ChangeEndCustomer.Execute(), Correlation Id: {0}, Initiating User: {1}",
                context.CorrelationId,
                context.InitiatingUserId);

            IOrganizationServiceFactory serviceFactory = executionContext.GetExtension<IOrganizationServiceFactory>();
            IOrganizationService service = serviceFactory.CreateOrganizationService(context.UserId);

            try
            {
                // TODO: Implement your custom Workflow business logic.
                Entity entity = null;

                if (context.InputParameters != null && context.InputParameters.Contains("Target") && context.InputParameters["Target"] is Entity)
                {
                    entity = (Entity)context.InputParameters["Target"];
                }
                else
                {
                    entity = service.Retrieve(context.PrimaryEntityName, ((Guid)context.PrimaryEntityId), new ColumnSet(true));
                }

                Guid ProjectID = new Guid();
                Guid EndCustomerID = new Guid();
                Guid ProgramID = new Guid();
                Guid NullCustomerID = new Guid();

                Entity RetrieveProject = service.Retrieve("ibiz_project", entity.Id, new ColumnSet(true));

                ProjectID = RetrieveProject.GetAttributeValue<Guid>("ibiz_projectid");
                EndCustomerID = RetrieveProject.GetAttributeValue<EntityReference>("ibiz_endcustomer").Id;

                ConditionExpression CheckProject = new ConditionExpression();
                CheckProject.AttributeName = "ibiz_project";
                CheckProject.Operator = ConditionOperator.Equal;
                CheckProject.Values.Add(ProjectID);

                FilterExpression FilterProject = new FilterExpression();
                FilterProject.Conditions.Add(CheckProject);

                QueryExpression queryProject = new QueryExpression("ibiz_program");
                queryProject.ColumnSet = new ColumnSet(true);
                queryProject.Criteria.AddFilter(FilterProject);

                EntityCollection resultProject = service.RetrieveMultiple(queryProject);

                if (resultProject.Entities.Any())
                {
                    foreach (Entity eProject in resultProject.Entities)
                    {
                        ProgramID = eProject.GetAttributeValue<Guid>("ibiz_programid");

                        if (EndCustomerID != null)
                        {
                            Entity ProgramToUpdate = new Entity("ibiz_program");
                            ProgramToUpdate["ibiz_programid"] = ProgramID;
                            ProgramToUpdate["ibiz_endcustomer"] = new EntityReference("account", EndCustomerID);
                            service.Update(ProgramToUpdate);
                        }
                        else
                        {
                            Entity ProgramToUpdate = new Entity("ibiz_program");
                            ProgramToUpdate["ibiz_programid"] = ProgramID;
                            ProgramToUpdate.Attributes.Add("ibiz_endcustomer", null);
                            service.Update(ProgramToUpdate);
                        }
                    }
                }
            }
            catch (FaultException<OrganizationServiceFault> e)
            {
                tracingService.Trace("Exception: {0}", e.ToString());

                // Handle the exception.
                throw;
            }

            tracingService.Trace("Exiting ChangeEndCustomer.Execute(), Correlation Id: {0}", context.CorrelationId);
        }
    }
}