Hi all, 
          I am begginer in salesforce.I have added below code in apex class.

"Insert failed. First exception on row 0; first error: FIELD_CUSTOM_VALIDATION_EXCEPTION, Attempt to de-reference a null object: []"
This type   of error.Please help me anyone.
 Apex class:
    AddPSProducts(newOppMap); 

Private void AddPSProducts(Map<Id,Opportunity> newOppMap){
        List<Opportunity> listOppsToAddProduct=new list<Opportunity>();
        List<id> listAccIds=new List<Id>();                
        List<id> listPSIds=new List<Id>();
        for(Opportunity opp:newOppMap.values()){        //Collecting the Accounts ids from the Triggered Opportunities
            if(opp.Project_Sheet__c!= NULL){
                listPSIds.add(opp.Project_Sheet__c);
                listAccIds.add(opp.AccountId);
            }              
        }
        
        //To avoid Unneccessary Executions.
        if(listPSIds.size() == 0) return;
        
        Map<Id,Account> customerMap=new Map<Id,Account>([select id,Delivery_Charged_for__c,Price_Book_ID__c from Account where id IN: listAccIds]);        
        Map<Id,Project_Sheet__c> psMap=new Map<Id,Project_Sheet__c>([SELECT id,Visit__r.Products__c,Site_Contract__c,Arrive__c,Depart__c,Visit__r.Quotation_Reference__c
                                                                     FROM Project_Sheet__c
                                                                     WHERE Id IN: listPSIds]);        
        for(Opportunity newOpp:newOppMap.values()){            
            if(psMap.containskey(newOpp.Project_Sheet__c) ){
                listOppsToAddProduct.add(newOpp);                
            }
        }     
         try{
            if(listOppsToAddProduct.size()>0){
                //Add Project Sheet Sheet Product to Opportunity
                addProduct=new OpportuntiyAutoProducts(psMap);
                System.debug('scMap========??'+scMap);
                listOLIs.addAll(addProduct.AddPSVisitProducts(customerMap,listOppsToAddProduct,prodMap,producsPBEMap,scMap)); 
            }
        }Catch(Exception e){
            String msg=e.getMessage();
            for(Opportunity o:newOppMap.values()){
                o.addError(msg);
          }
        }       
    } 

Advance in Thanks