My case is:
When a user selects a product, I want my trigger to look at the add_for__c value on that product and if it equals 0 then it should look for all products where the add_for__c field value is equal to the lsrv_pk__c value of the initial product selected.

I'm still very new to writing triggers and only just starting the databese and SQL portion of my education, but need this trigger for work ASAP so any help would be appreciated!

I found some sample code through my research on this need and have attempted to re-work it to my need and managed to get the trigger to save, but when I test it, I'm getting this error: 

Product_Auto_Add: execution of BeforeInsert
caused by: System.NullPointerException: Attempt to de-reference a null object
Trigger.Product_Auto_Add: line 40, column 1

I'm looking for help resolving this issue and also to ensure I've created my trigger to execute what I need it to.  I have no loops in there...do I need a loop to ensure it gets ALL products that have the matching value I mentioned above?

Here's the trigger:
 

trigger Product_Auto_Add on OpportunityLineItem (before insert) {
    
    List<OpportunityLineItem> oliList = new List<OpportunityLineItem>(); 
    
    List<String> lstProduct = new List<String>();
    
    for(OpportunityLineItem optLineItem: Trigger.new)
    {
        if(optLineItem.add_for__c == '0')
        {
            lstProduct.add(optLineItem.add_for__c);
        }
    }
    
    //retrieve values of Product Selected
    List<OpportunityLineItem> lstOpptyLineItems = [SELECT OpportunityId, Name, PricebookEntryId, Quantity, add_for__c, lsrv_pk__c, UnitPrice
                                                   FROM    OpportunityLineItem
                                                   WHERE   Id = :Trigger.new 
                                                   AND add_for__c IN:lstProduct];
    
    //create a map which contains Product Name and OpportunityLineItem
    Map<String, OpportunityLineItem> mapOpptyLineItem = new Map<String, OpportunityLineItem>();
    for(OpportunityLineItem item:lstOpptyLineItems)
    {
        mapOpptyLineItem.put(item.lsrv_pk__c, item);
    }
    
    //retrieve PriceBookEntry of the Product B, this is most important
    PricebookEntry pbeProduct2 = [SELECT Id, Pricebook2Id, UnitPrice, Name, add_for__c
                                  FROM PricebookEntry 
                                  WHERE Pricebook2Id IN 
                                  (SELECT Id FROM PriceBook2 
                                   WHERE Name = 'Selectcom Telecom Pricing') 
                                  LIMIT 1];
    
    
    //retrieve Product A item from the map.         
    OpportunityLineItem itemProductA = mapOpptyLineItem.get('Product A');
    
    if(itemProductA.lsrv_pk__c == pbeProduct2.add_for__c){
        
        
        oliList.add(new OpportunityLineItem(
            OpportunityId = itemProductA.OpportunityId, 
            PricebookEntryId = pbeProduct2.Id,
        	Quantity = itemProductA.Quantity,
        	UnitPrice = pbeProduct2.UnitPrice)
                   );
        
        insert oliList;
    }
}

FYI Line 40 is:

if(itemProductA.lsrv_pk__c == pbeProduct2.add_for__c){

Thank you!