Attempt to de-reference null object (Custom File Import)

Question

The requirement is to import a tab separated text file and updated the records matching with some statuses then display success to for failure with the list of records.

I wrote the following VF and apex class

<apex:page standardController="Card_Request__c" extensions="CarerCardImportController" readOnly="false">
    <apex:form >
        <apex:pagemessages />
        <apex:pageBlock>
            <apex:pageBlockSection columns="2">
              <apex:inputFile value="{!csvFileBody}"  filename="{!csvAsString}"/>
                <br/>
                <br/>
              <apex:commandButton value="Import Cards" action="{!importFile}"/>
            </apex:pageBlockSection>

            <apex:pageBlockTable value="{!updatedCardsList}" title="" var="cr">
                <apex:column value="{!cr.card.Carers_Card_Number__c}"/>
                <apex:column value="{!cr.card.Card_Status__c}"/>
                <apex:column value="{!cr.card.Carer_Sub_Status__c}"/>
                <apex:column value="{!cr.card.CC_Expiry_Date__c}"/>
                <apex:column value="{!cr.result}"/>
            </apex:pageBlockTable>

        </apex:pageBlock>
    </apex:form>
</apex:page>

Apex class look like following

public with sharing class CarerCardImportController{

    public transient Blob csvFileBody {get; set;}
    public String csvAsString {get; set;}
    public String[] csvFileLines {get; set;}
    public List<Card_Request__c> cardToUpdate {get; set;}
    public static String INITIAL_STATUS = 'Issued';
    public static Map<Id, Card_Request__c> cardsUpdated {get; set;}
    public static List<CardWrapper> updatedCardsList {get; set;}
    public static Set<Id> cardIds {get; set;}

    public CarerCardImportController(){
        csvFileLines = new String[]{};
        cardToUpdate = new List<Card_Request__c>(); 
    }

    public CarerCardImportController(ApexPages.StandardController controller){
        cardsUpdated = new Map<Id, Card_Request__c>();
        updatedCardsList= new List<CardWrapper>();
        csvFileLines = new String[]{};
        cardToUpdate = new List<Card_Request__c>();
        cardIds = new Set<Id>();
    }


    public class CardWrapper{
        public Card_Request__c card {get;set;}
        public String result {get;set;}

        public CardWrapper(Card_Request__c card, String result){
            this.card = card;
            this.result = result;
        }
    }

    public void importFile(){
        try{
           csvAsString = csvFileBody.toString();
           csvFileLines = csvAsString.split('\n'); 
           //get all the card requests that approved before 
           List<Card_Request__c> approvedCards = [SELECT Id, Carers_Card_Number__c, Card_Status__c, DateIssued__c, Carer_Sub_Status__c 
                                        FROM Card_Request__c WHERE (Card_Status__c = 'Issued' AND Carer_Sub_Status__c = 'Waiting - Print Receipt')];

           cardToUpdate = new List<Card_Request__c>();
           //compare the cards with the received CSV
            for(Card_Request__c card : approvedCards){

                for(Integer i = 1; i < csvFileLines.size(); i++){
                    String[] csvRecordData = csvFileLines[i].split('\t');
                    if(card.Carers_Card_Number__c == csvRecordData[0]){
                        System.debug(csvRecordData[0]);
                        card.Card_Status__c = 'Active';
                        card.Carer_Sub_Status__c = 'Sent';
                        String cleanedCell = csvRecordData[1].trim();
                        String[] splitDate = String.valueOf(cleanedCell).split('/');

                        Date parsedIssuedDate = Date.newInstance(Integer.valueOf(splitDate[2]), Integer.valueOf(splitDate[1]), Integer.valueOf(splitDate[0]));
                        card.DateIssued__c = parsedIssuedDate;
                        cardToUpdate.add(card);
                        cardIds.add(card.Id);
                    }
                }   
            }
        }
        catch(Exception e){
            ApexPages.Message errorMessage = new ApexPages.Message(ApexPages.severity.ERROR,'An error has occured while importin data Please make sure input csv file is correct');
            ApexPages.addMessage(errorMessage);     
            System.debug(e);
            System.debug(e.getMessage());
            System.debug(e.getCause());
            System.debug(e.getLineNumber());
        }

        try{
            Database.SaveResult[] results = Database.update(cardToUpdate);

                cardsUpdated = new Map<Id,Card_Request__c>([SELECT Id, Card_Status__c, Carer_Sub_Status__c, Application__c, Carers_Card_Number__c, CC_Expiry_Date__c
                                FROM Card_Request__c WHERE Id IN :cardIds]);                 

            for(Integer i = 0; i < results.size(); i++){
                if(results[i].isSuccess()){
                    Card_Request__c card = cardsUpdated.get(results.get(i).getId());
                    updatedCardsList.add(new CardWrapper(card, 'SUCCESS'));
                }else if (!results.get(i).isSuccess()){
                    String errors = '';
                    for(Database.error err : results.get(i).getErrors()){
                        errors += err.getStatusCode() + ': ' + err.getMessage() + '\n';
                    }
                    Card_Request__c card = cardsUpdated.get(cardToUpdate.get(i).Id);
                    updatedCardsList.add(new CardWrapper(card, errors));
                }
            }
        }
        catch(Exception e){
            ApexPages.Message errorMessage = new ApexPages.Message(ApexPages.severity.ERROR,'An error has occured while importin data');
            ApexPages.addMessage(errorMessage);     
            System.debug(e);
            System.debug(e.getMessage());
            System.debug(e.getCause());
            System.debug(e.getLineNumber());
        }  
    }
}

it come up with attempt to de-reference a null object error at line where i'm adding the id to cardIds Set can some please help to get rid of this problem.

Answer 1

You haven't instantiated the cardIds set in context of the importFile method. Add

cardIds = new Set<Id>(); 

somewhere before or in your try block (around line 37).