Invocable Method returning List of List - Attempt to Dereference a null object

Question

I'm working with cloud flow designer and having some issues with the Apex class callout. I know I will need to return a List<List<CustomSobject>> from the invocable method in order to process the collection. But I am stuck overcomplicating the code in order to get what I would normally use a List for into the right format to return. I probably need an additional for loop...? If I only use a List<CustomSobject> the Flow will not be able to process it as a collection returning multiple records. Below is my method, this current version compiles but the flow will throw an "Attempt to de-reference a null object" error.

@InvocableMethod(label = 'Services Available')
public static List<List<Create_Product__c>> getServices(List<Address__c> addressInput){
    //created to hold return list of lists
    List<List<Create_Product__c>> wrapper = new List<List<Create_Product__c>>();

    //The data set to be returned
    List<Create_Product__c> productsAvailable = [SELECT Product_Code__c, Starting_Price__c FROM Create_Product__c];

    for (Create_Product__c products: productsAvailable ){
        List<Create_Product__c> responseList = new List<Create_Product__c>();

        responseList[0].Product_Code__c = products.Product_Code__c;
        responseList[0].Starting_Price__c = products.Starting_Price__c;

        wrapper.add(responseList);
    }

    return wrapper;
}

Answer 1

List<Create_Product__c> responseList = new List<Create_Product__c>(); collection is declared inside for loop and without adding an element to collection, 0th element is being accessed, which will throw List out of bound exception.

You can probable try modifying code to

List<Create_Product__c> productsAvailable = [SELECT Product_Code__c, Starting_Price__c FROM Create_Product__c]; wrapper.addAll(productsAvailable);

Reference Links
List Class, addAll method
System.ListException: List index out of bounds: 0

Answer 2

If you're getting a null object error from within your flow, I'm going to guess that could be similar to a null pointer exception (depends on what's considered an object in the flow).

That tells me that in your loop below, either Product_Code__c or Starting_Price__c could be empty or null. It would seem that you need to add a check to your code to make certain that both of those values are populated before adding responseList[0] (NOT responseList) to your wrapper.

You're not iterating withing each Product_Code__r and Starting_Price__r to see if a list of either of those exists (I assume that's possible, but perhaps not), instead only checking for a single value. As such, you only want to add responsList[0] provided both the product_code and the starting_price have values.

for (Create_Product__c products: productsAvailable ){
    List<Create_Product__c> responseList = new List<Create_Product__c>();

    responseList[0].Product_Code__c = products.Product_Code__c;
    responseList[0].Starting_Price__c = products.Starting_Price__c;

    if(!products.Product_Code__c.isEmpty() && !products.Starting_Price__c.isEmpty()){
        wrapper.add(responseList[0]);
    }
}