Hello,

I am relatively new to Apex and Visualforce and need some guidance.

I have 2 objects: Billing Account and Line Items. For every Account, there can be multiple Billing Accounts each of which can have multiple Line Items. I want to return a List from a function that will give me all the Line Items of all Billing Accounts in one single list. This code compiles but on running the code, it gives me an error

System.NullPointerException: Attempt to de-reference a null object at line

totallines.addAll(lines);

Here is my apex class snippet: 

List<Line_Items__c> l, lines, totallines;
List<Billing_Account__c> billacc;    
public List<Line_Items__c> getLineItems()
    {
        if(invoice.Billing_Account__c != null) 
        {
            l = [select id, name, Plan__c, Rate__c, Quantity__c, Total_Value__c, Activation_Date__c, Plan_Name__c, Street__c, 
                 Unit__c, City__c from Line_Items__c where Service_Account__c =: invoice.Billing_Account__c and Status__c =: 'Active'];
            
            return l;
                       
        }
        else
        { 
            billacc = [select id, Bill_Street__c, Bill_Unit_Number__c, Bill_City__c, Bill_State__c, Bill_Postal_Code__c from Billing_Account__c 
                                    where Account__c =: invoice.Account__c];
            
            integer z = 0;  
            if(billacc.size() > 0)
            {                      
                for(integer k =0; k < billacc.size(); k++)
                {
                   lines = [select id, name, Plan__c, Rate__c, Quantity__c, Total_Value__c, Activation_Date__c, Plan_Name__c, 
                   Street__c, Unit__c, City__c from Line_Items__c                   where Service_Account__c =: billacc[k].id and Status__c =: 'Active'];

                   for( integer h = 0; h < lines.size(); h++)
                   {
                       totallines.addAll(lines);
                       z++;
                   }

                }
                 
                 return totallines;
             }
             
             return null;
             
         } 
                                      
    }