Getting System.DMLException in test class for below trigger. Please help me to solve it.

Trigger:- 

trigger OpportunityHowell on Opportunity (after Insert,after Update) {
   
  List<Opportunity> listOpportunitiesToUpdate = new List<Opportunity>();
  Set<Id> allOids = new Set<Id>();
  for(Opportunity o: Trigger.new){
    if(o.Multiplier__c != null && o.Multiplier__c != trigger.oldMap.get(o.id).Multiplier__c){
      allOids.add(o.id);
    }
  }
  listOpportunitiesToUpdate = [select id, (SELECT Id,Dollar_Sales_Howell__c from OpportunityLineItems) from Opportunity where id in :allOids];
  for(Opportunity o:listOpportunitiesToUpdate){
    o.Sum_of_Dollar_Sales_Howell__c = 0;
    for(OpportunityLineItem oli: o.OpportunityLineItems)
    Try{
      o.Sum_of_Dollar_Sales_Howell__c += oli.Dollar_Sales_Howell__c;
      }
    Catch(Exception e)
    {
  }
  if(listOpportunitiesToUpdate.size()>0)   
    update listOpportunitiesToUpdate;    
    }
  }

Test class:-

@isTest
public class OpportunityHowellTest {

    public static testMethod void TestOppty(){
        
         Set<Id> allOids = new Set<Id>();
         MHC2__Project__c p = new MHC2__Project__c(Name='Test Project',MHC2__MHC_Project_Bid_Date__c=System.Today(),MHC2__MHC_Target_Bid_Date__c=System.Today());
        insert p;
        Account a = new Account(Name='Test Account');
        insert a;
        
        Opportunity op = new Opportunity();
        op.Name = 'My Test Opportunity';
        op.Multiplier__c = 2;
        op.Sum_of_Dollar_Sales_Howell__c = 0;
        op.Bid_Date_Time__c = System.Now().addDays(30);
        op.CloseDate = System.Now().addDays(45).date();
        op.StageName = 'Bidding';
        op.AccountId = a.Id;
        op.Bid_Type__c = 'Lump Sum';
        op.Project__c = p.Id;
        insert op;
        
        op.Multiplier__c = 5;
        
        update op;
     
        
       OpportunityLineItem oli = new OpportunityLineItem();
        oli.Quantity = 1.0;
        oli.OpportunityId = op.Id;
        oli.Invoiced__c = false;
        insert oli; 
  }
}

Error:-

System.DmlException: Insert failed. First exception on row 0; first error: CANNOT_INSERT_UPDATE_ACTIVATE_ENTITY, OpportunityHowell: execution of AfterInsert

caused by: System.NullPointerException: Attempt to de-reference a null object

Trigger.OpportunityHowell: line 6, column 1: []