trigger ContractinsertandUpdate on Contract (before insert, before update,after insert, after update) {

    Map< Id , Id > OppIdToContractId = new Map< Id , Id >();
    List<OpportunityLineItem> OpportunityLineItemList = new List<OpportunityLineItem>();
    Set<ID> oppIds = new Set<ID>();
    List<Contract_Items__c> OldCIs = new List<Contract_Items__c>();
    Map<string,Contract_Items__c> mapOliItems =new  Map<string,Contract_Items__c>();

    if(Trigger.isAfter)
    {
  
        for(Contract contract :trigger.new){
            
            OppIdToContractId .put(contract .Opportunity__c,contract .Id);
            //oppIds.add(contract .Opportunity__c);  
        }

        OpportunityLineItemList = [select Id,Name,Product2.Name,Product2.Id,Product2.ProductCode, Quantity,UnitPrice,
                   CurrencyIsoCode,OpportunityId from opportunityLineItem
                   where OpportunityId IN :OppIdToContractId.keySet() ];
        
        
        OldCIs = [select Id, name,Item__c from Contract_Items__c where contract__c IN :trigger.new ];
        
        for(Contract_Items__c oItems :OldCIs ){
          
            mapOliItems .put(oItems .Item__c ,oItem);
        }
              if(OldCIs .size() == 0){
            if(OpportunityLineItemList .size()>0){
			    list<Contract_Items__c> newItems = new List<Contract_Items__c>();

                for(opportunityLineItem oli :OpportunityLineItemList ){
                    Contract_Items__c CI = new Contract_Items__c();
                    CI .Name = oli.Name;
                    CI .Offering__c = oli.Product2.Id;
                    CI .Contract__c = OppIdToContractId .get(oli.OpportunityId);
                    CI .Quantity__c = oli.Quantity;
                    CI .Price__c= oli.UnitPrice;
                    CI .Item__c = oli.Product2.ProductCode;
                    newItems .add(CI ); 
                }
				if(newItems.size() > 0)
					insert newItems ;
            }
        }else{
			list<Contract_Items__c> existingItems = new List<Contract_Items__c>();

            for(opportunityLineItem oli22 :OpportunityLineItemList ){
              
                if(mapOliItems .containsKey(oli22.Product2.ProductCode)){
                    Contract_Items__c newCI1 = mapOliItems .get(oli22.Product2.ProductCode);
                    newCI1 .Name = oli22.Name;
                    newCI1 .Offering__c = oli22.Product2.Id;
                    newCI1 .Quantity__c = oli22.Quantity;
                    newCI1 .Price__c= oli22.UnitPrice;
                    newCI1 .Item__c = oli22.Product2.ProductCode;
                    existingItems.add(newCI1 );    
                }else{
                    Contract_Items__c newCi = new Contract_Items__c();
                    newCi .Name = oli22.Name;
                    newCi .Offering__c = oli22.Product2.Id;
                    newCi .Contract__c = OppIdToContractId .get(oli22.OpportunityId);
                    newCi .Quantity__c = oli22.Quantity;
                    newCi .Price__c = oli22.UnitPrice;
                    newCi .Item__c = oli22.Product2.ProductCode;
                    existingItems.add(newCi );
                }
            }
           
		   if(existingItems.size() > 0)
              upsert existingItems ; //(Here i'm facing error)
        }
        
        
    }
}