Recieving the error (NullPointerException) Unable to save resoure(s), when I try to compile the code below. 

Any guidance would be greatly appreciated. Thanks in advance.

public with sharing class TestUpDtCnt {
	
	static testMethod void ActUpdateTest(){
	  //create the test account	  
	  Account a = new Account(Name='My Test Account',
	  industry='corporate');
      insert a;
 	  
 	  //create the test contact
 	  Contact c = new Contact(
 	  FirstName='Tester10101',
 	  LastName='Tester10101',
 	  CompanyInterest__c='NetSim',
 	  accountid = a.id);
 	  insert c;
 	  
 	  Set<String> accSet = new Set<String>();	
      List<Contact> cnts = new List<contact>();
       
      for (Account a2 : [select id from Account where name = 'My Test Account'] ){
       	accSet.add(a2.id);
      }
       
      cnts = [Select ID from Contact where accountid in:accSet];
      
      try{  
      for (Account acct : [select id from Account where id in:accSet]){
       for(Contact ct:cnts){	 
        if (acct.Type == 'Customer - Netsim' ||  acct.Type =='Customer - NetsimOD'){
          ct.Customer_Type__c = 'Former';
        }
        update cnts;
        }
      }
      }catch (System.DmlException e){
         System.debug('we caught a dml exception: ' + e.getDmlMessage(0)); 
      }   
     }
}