Plugin Error

namespace Mangazeya_Plugins
{
    using System;
    using System.ServiceModel;
    using Microsoft.Xrm.Sdk;
    using Microsoft.Xrm.Sdk.Query;

    public class PostTisa_ArticleUpdateFurnishSum : Plugin
    {
        public PostTisa_ArticleUpdateFurnishSum()
            : base(typeof(PostTisa_ArticleUpdateFurnishSum))
        {
            base.RegisteredEvents.Add(new Tuple<int, string, string, Action<LocalPluginContext>>(40, "Update", "tisa_article", new Action<LocalPluginContext>(ExecutePostTisa_ArticleUpdateFurnishSum)));

        }

        protected void ExecutePostTisa_ArticleUpdateFurnishSum(LocalPluginContext localContext)
        {
            if (localContext == null)
            {
                throw new ArgumentNullException("localContext");
            }

            IPluginExecutionContext context = localContext.PluginExecutionContext;
            IOrganizationService service = localContext.OrganizationService;
           
            if (context.InputParameters.Contains("Target") && context.InputParameters["Target"] is Entity)
            {
                // Obtain the target entity from the input parmameters.
                Entity entity = (Entity)context.InputParameters["Target"];
                if (entity.LogicalName == "tisa_article")
                {
                    if (context.MessageName.ToUpper() == "UPDATE")
                    {
                        var pricewoFurnish = entity.Attributes["tisa_price"];
                        ColumnSet attributes = new ColumnSet(new string[] { "tisa_code", "tisa_addressid", "tisa_pricefurnishstandart", "tisa_pricefurnishdesign", "tisa_platformnumber", "tisa_quantity" });
                        var tisa_article = service.Retrieve(entity.LogicalName, entity.Id, attributes);

                        var tisa_addressid = ((EntityReference)tisa_article["tisa_addressid"]).Id;

                        string fetchxml = @"
                        <fetch distinct='false' mapping='logical' output-format='xml-platform' version='1.0'>
                            <entity name='tisa_costfurnishvariation'>
                                    <attribute name='tisa_costfurnishvariationid'/>
                                    <attribute name='tisa_isfurnishprice"+ tisa_article["tisa_platformnumber"] + @"'/>
                                    <attribute name='tisa_tisa_platformnumber" + tisa_article["tisa_platformnumber"] + @"'/>
                                    <attribute name='tisa_mainfurnish'/>
                                    <attribute name='tisa_addressid'/>
                                <order descending='false' attribute='tisa_addressid'/>
                                <filter type='and'>
                                    <condition attribute='tisa_addressid' value='" + tisa_addressid + @"' operator='eq'/>
                                </filter>
                            </entity>
                        </fetch>";

                        EntityCollection result = service.RetrieveMultiple(new FetchExpression(fetchxml));
                        foreach (var c in result.Entities) 
                        {
                            if (result != null && result.Entities.Count > 0) 
                            {
                                int value = ((OptionSetValue)c["tisa_mainfurnish"]).Value;

                                if (value == 1) 
                                {
                                    entity["tisa_pricefurnishstandart"] = ((Convert.ToDecimal(c["tisa_isfurnishprice" + tisa_article["tisa_platformnumber"]])) * (Convert.ToDecimal(tisa_article["tisa_quantity"]))) + Convert.ToDecimal(entity.GetAttributeValue<Money>("tisa_price").Value); ;
                                }
                                else if(value == 2)
                                {
                                    entity["tisa_pricefurnishdesign"] = ((Convert.ToDecimal(c["tisa_isfurnishprice" + tisa_article["tisa_platformnumber"]])) * (Convert.ToDecimal(tisa_article["tisa_quantity"]))) + Convert.ToDecimal(entity.GetAttributeValue<Money>("tisa_price").Value);
                                }
                            }
                        }

                        service.Update(entity);
                    }
                }
            }
        }
    }
}