System.NullPointerException: Attempt to de-reference a null object - error while updating

Question

I am trying to update a account using the below method called from an after update, after insert trigger. However I am getting an error - "Attempt to de-reference a null object" while using Account acc = null; in the below code..AND if I use Account acc = new Account() , I am getting the error - missing_argument id not specified in an update call in salesforce.Kindly assist.

   Trigger:              
   trigger testTrigg on CustObj(after insert,after update) 
   {
       for (CustObj cr1 : Trigger.new)
       {
           myClass.updateAccount(cr1);
       }
   }
 Class:
  public static myClass{
    public static void updateAccount(CustObj cr)
    {   
       List<Account> acclist = new List<Account>();
       for (Account ac : soql query)
      {               
         acclist.add(ac);
      }    
      Account acc = null;
      if (acclist.size() > 0)
      {
         acc = acclist[0];
         acc.Name = cr.Account_Name__c; 
         acc.Other_title_information__c = cr.JobTitle__c ;   
      }
     Database.SaveResult sr= Database.update(acc);
    }
  }

Answer 1

This change should stop the exception because it ensures that the update is only done when there is an Account value to update:

if (acclist.size() > 0)
{
    Account acc = acclist[0];
    acc.Name = cr.Account_Name__c; 
    acc.Other_title_information__c = cr.JobTitle__c ;   
    Database.SaveResult sr = Database.update(acc);
    // Do something with sr otherwise why assign it?
}

But your design is doing a query and an update per CustObj and so will break when large bulk updates of CustObj happen. Google about and apply trigger bulkification techniques.