// This is the method in Test class

static testMethod void testNotUltimateButLocalClient() {
        // specify local client only and pull the ultimate client by traversing hierarchy
        System.debug('Testing insert case where Ultimate Parent Client IS NOT populated but Local Client IS populated which has Parent Account tied to it.');
        Account ultimateAccount= new Account(name='TestOverrideClient Thirteen',BillingStreet='TestOverrideThirteenSt', Client_Short_Name__c ='ulttiCL',
                Phone='123454321',BillingState='ca', BillingPostalCode='95456', BillingCountry='US', BillingCity='TestOverrideThirteenCity', Ultimate_Parent__c = true);
        insert ultimateAccount;
        
        Account localAccount  = new Account(name='TestOverrideClient Fourteen',BillingStreet='TestOverrideFourteenSt', Client_Short_Name__c ='localCL', Ultimate_Parent_Client__c =ultimateAccount.id,
                        BillingState='ca', BillingPostalCode='95457', BillingCountry='US', BillingCity='TestOverrideFourteenCity', Ultimate_Parent__c = false,phone='111-111-1111');
        insert localAccount;
        
        Opportunity ultimateOpp = new Opportunity(Local_Client__c=localAccount.Id, Name='testoppfornamechange',Solution_Type__c='Standard',
                        Opportunity_Short_Description__c='test desc',Active_Latent__c='Active',Opportunity_Type__c='parent',
                        StageName='plan & engage',CloseDate=System.today()+2);
        
        insert ultimateOpp;
        
        Opportunity ulOpp = [select id, name, accountId, Local_Client__c from Opportunity where id =:ultimateOpp.Id];
        system.assertEquals('TestOver - 0 - test desc', ulOpp.Name);
        system.assertEquals(localAccount.id, ulOpp.Local_Client__c);
        system.assertEquals(ultimateAccount.Id , ulOpp.accountId);
        
        System.debug('Testing update case where Ultimate Parent Client IS NOT populated but Local Client IS populated which has Parent Account tied to it.');
        Account ultimateAccount2= new Account(name='TestOverrideClient Fifteen',BillingStreet='TestOverrideFifteenSt', Client_Short_Name__c ='ulttiC2',
                BillingState='ca', BillingPostalCode='95458', BillingCountry='US', BillingCity='TestOverrideFifteenCity', Ultimate_Parent__c = true,phone='111-111-1111');
        insert ultimateAccount2;
        
        Account localAccount2  = new Account(name='TestOverrideClient Sixteen',BillingStreet='TestOverrideSixteenSt', Client_Short_Name__c ='localC2',Ultimate_Parent_Client__c = ultimateAccount2.id,
                        BillingState='ca', BillingPostalCode='95459', BillingCountry='US', BillingCity='TestOverrideSixteenCity', Ultimate_Parent__c = false,phone='111-111-1111');
        insert localAccount2;
        ultimateOpp.AccountId = null;
        ultimateOpp.Local_Client__c = localAccount2.id;
        update ultimateOpp;
        Opportunity ulOpp2 = [select id, name, accountId, Local_Client__c from Opportunity where id =:ultimateOpp.Id];
        system.assertEquals('TestOver - STD - 0 - 0 - 0 - test desc', ulOpp2.Name);
        system.assertEquals(localAccount2.id, ulOpp2.Local_Client__c);
        system.assertEquals(ultimateAccount2.Id , ulOpp2.accountId);
        
    }

This is the code mentioned in Stack Trace

trigger OpportunityBeforeTrigger on Opportunity (before insert,before update) {
   Set<Id> parentOppIdSet = new Set<Id>();
   Set<Id> validOppIdSet = new Set<Id>();
    for(Opportunity opp: Trigger.new) {
        if(opp.Parent_Opportunity__c != null) {
            if(opp.Opportunity_Type__c == 'Parent')
                opp.addError('Parent should not have another opportunity as parent.');
              
            parentOppIdSet.add(opp.Parent_Opportunity__c);            
        } else {
            if(opp.Opportunity_Type__c == 'Child')
                opp.addError('Please enter a Parent Opportunity. ');
            validOppIdSet.add(opp.Id);
        }        
    }
    System.debug('valididset'+validoppidset);
    if(validOppIdSet != null) {
        try {
            Map<Id, Opportunity> childOppMap = new Map<Id, Opportunity>([Select Id, Name, Parent_Opportunity__c from Opportunity where Parent_Opportunity__c IN: validOppIdSet limit 30]);
            Set<Id> errorOppIdSet = new Set<Id>();
            List<OpportunityLineItem> oppLineItemList = new  List<OpportunityLineItem>();
            if(childOppMap != null && childOppMap.size() > 0) {
                Set<Id> childOppIdSet = childOppMap.keySet();
                try{
                    oppLineItemList = [Select Id, OpportunityId from OpportunityLineItem where OpportunityId IN: childOppIdSet limit 50];
                }
                catch(Exception e){
                }
                if(oppLineItemList != null && oppLineItemList.size() > 0) {
                    for(Opportunity childOpp: childOppMap.values()) {
                        Id childOppId = childOpp.Id; 
                        Boolean isOppLineItemExists = false;
                        for(OpportunityLineItem oppLineItem : oppLineItemList) {
                            if(childOppId == oppLineItem.OpportunityId) {
                                isOppLineItemExists = true;
                                break;
                            }
                        }