I created a new partner portal. When the partner user creates a new lead and clicks 'save' they recieve the folloing error:

Error: Invalid Data. 
Review all error messages below to correct your data.
Apex trigger UpdateReferrals caused an unexpected exception, contact your administrator: UpdateReferrals: execution of AfterInsert caused by: System.NullPointerException: Attempt to de-reference a null object: Trigger.UpdateReferrals: line 33, column 1

This is only occuring with this new partner portal.  I logged in into an old partner portal user and they are able to create and save leads with this error occuring. 

See Apex Trigger. 

trigger UpdateReferrals on Lead (after insert) {

  Set<Id> createdByIds = new Set<Id>();
  List<Id> referralIds = new List<Id>();
  
  for (Lead l : trigger.new){
    createdByIds.add(l.CreatedById);
  }
  
  // Return all users that created Leads in this trigger
  Map<Id, User> creator = new Map<Id, User>([Select Contact.Divisions__c, UserType FROM User WHERE Id IN :CreatedByIds]);
  
  // Find all of the PowerPartner users who created a Lead in this trigger
  for (Lead l : trigger.new){
    if (creator.get(l.CreatedById).UserType == 'PowerPartner'){
      referralIds.add (l.Id);
    }
  }      
  
  // Get all of the Leads owned by PowerPartner users discovered above
  List<Lead> referrals = new List<Lead>([SELECT CreatedById, Name, Company, ReplyTo__c FROM Lead WHERE ID in :referralIds]);
  
  // Loop through all of those Leads
  for (Lead l : referrals){
    
    // Set the Partner Sales Rep to the creator of the Lead
    l.Partner_Sales_Rep__c = l.CreatedById;
    
    // Get the divisions of the Partner Sales Rep (From its contact record)
    l.Divisions__c = creator.get(l.CreatedById).Contact.Divisions__c;
    
    // Set the singular division on the Lead if the Partner only has one division
    if (creator.get(l.CreatedById).Contact.Divisions__c.indexOf(';', 0) == -1){
      l.Division__c = creator.get(l.CreatedById).contact.Divisions__c;
    }

  }
/*
  for (Lead p : referrals){
    Messaging.SingleEmailMessage mail = new Messaging.SingleEmailMessage();
    mail.setTargetObjectId(p.CreatedById);
    mail.setReplyTo(p.ReplyTo__c);
    mail.setSenderDisplayName(p.ReplyTo__c);
    mail.setSaveAsActivity(false);
    mail.setBccSender(false);
    mail.setUseSignature(false);
    mail.setSubject('Thank you for the referral!');
    mail.setHtmlBody('<p>We will review the referral:</p>'+
    '<p>Contact Name: ' + p.Name + '</br>' +
    '<p>Company Name: ' + p.Company + '</p>' +
    '<p>We will follow-up with you shortly.</p>' +
    '<p>'+ p.ReplyTo__c + '</p>');
    Messaging.sendEmail(new Messaging.SingleEmailMessage[] { mail });        
  }
*/
  update referrals;

}