Im getting the error 'null argument for JSONGenerator.writeStringField()' when I call the below class from Visual Page.

Visualforce Page Code:

<apex:page standardController="Lead"   extensions="CreateSalesOrder" action="{!getRedir}">  
<apex:form >
<apex:pageBlock >
<apex:pageBlockSection title="Error Messages" collapsible="false" columns="1">
  <apex:pageMessages id="errorMessages" />
  <apex:outputLink value="/{!$CurrentPage.parameters.id}">&lt; Return to Lead Page</apex:outputLink>

Below is the Apex Class

public with sharing class CreateSalesOrder {
private Boolean Failed = false;
private Integer responseCode;  
private String leadId;
public CreateSalesOrder(ApexPages.StandardController controller) {
Http h = new Http();
String url ='http://xxx.stage.com/salesforce/api';
this.leadId =  ApexPages.currentPage().getParameters().get('ID');
HttpRequest req = buildWebServiceRequest(url,  this.leadId);
HttpResponse res = invokeWebService(h,req);
System.Debug('RESPONSE: ' + res.getBody().trim());   
System.debug('HTTP Response Code: ' + res.getStatusCode());
System.debug('Response Body: ' + res.getBody());   
if (res.getStatusCode()== 201){
    this.Failed =true;
    this.Failed =false;            

public HttpRequest buildWebServiceRequest(String url, String ids) {
  //Build HTTP Request object
    JSONGenerator gen = JSON.createGenerator(true);    

    List <Lead> Leads =  [SELECT Company,FirstName,LastName, Street, City, State, PostalCode, Phone, Email FROM Lead WHERE id=:Ids];
    for(Lead c:Leads){           
        gen.writeStringField('companyName', c.Company);
        String jsonS = gen.getAsString();

        // Sending the http body with JSON 
            HttpRequest req = new HttpRequest();
            req.setHeader('Content-Type', 'application/json;charset=UTF-8');
            req.setHeader('Accept', 'text/html');
            return req;

public HttpResponse invokeWebService(Http h, HttpRequest req) {
  //Invoke Web Service
   HttpResponse res = h.send(req); 
         return res;        

public PageReference getRedir() {

if (!this.Failed) {
     ApexPages.addMessage(new ApexPages.Message(ApexPages.Severity.ERROR, 'Sorry.  We got ' + responseCode + ' ' + this.leadId +'  Response Code from atlas.  Please try again later.'));
  //PageReference newPage = new PageReference('http://www.google.com');
  //return newPage;
  return null;
} else {
  PageReference newPage = new PageReference('http://xxx.stage.com/' + this.leadId);
  return newPage;
private final ApexPages.StandardController controller;

enter image description here

can you please let me what is the issue on above code?


You can't write a null value with the writeStringField function; instead, if the field is null, you have to use the writeNullField function.

That means your code needs to look more like:

Map<String, SobjectField> fieldMap = new Map<String, SobjectField> {
  'companyName' => Lead.Company, 'firstName' => Lead.FirstName, 
  'lastName' => Lead.LastName };
for(Lead c:Leads){
    for(String key: fieldMap.keyset()) {
        String value = (String)c.get(fieldMap.get(key));
        if(value == null) {
        } else {
            gen.writeStringField(key, value);

Honestly, it'd be a lot easier to just write the code using a generic map instead:

String jsonS;
for(Lead c:Leads){
    jsonS = JSON.serialize(
        new Map<String, String> {
            'companyName' => c.Company,
            'firstName' => c.FirstName,
            'lastName' => c.LastName

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