i need helping finding why i am getting that error. and also is there a way i can identify the actual record causing it instead of the output just saying that error ?

public void doPhoneComparison(){
		
		try{
		
		// sf contact phone numbers
		map<string,string> map1 = new map<string,string>();
		
		// phone numbers obj phone numbers
		map<string,string> map2 = new map<string,string>();
		
		// get all contact phone numbers, there is going to be over 7000
		integer i=0;
		
			ApexPages.addMessage( new ApexPages.Message(
    		ApexPages.Severity.INFO  , datetime.now() +' checking phone number field...'));

        	ApexPages.addMessage( new ApexPages.Message(
    		ApexPages.Severity.INFO  , datetime.now() +' checking external phone numbers...'));

  		// get all 'phone number' obj phone numbers
  		for(Phone_Numbers__c p : [select phone_number__c from Phone_Numbers__c]){
  			
  			p.phone_number__c = p.phone_number__c.replaceAll('-','');
			p.phone_number__c = p.phone_number__c.replaceAll('\\(','');
			p.phone_number__c = p.phone_number__c.replaceAll('\\)','');
			p.phone_number__c = p.phone_number__c.replaceAll(' ','');

              map2.put(p.phone_number__c,p.phone_number__c);
        }
        
        	ApexPages.addMessage( new ApexPages.Message(
    		ApexPages.Severity.INFO  , datetime.now() +' comparing phone numbers...'));
  
       // compare the two numbers
         for(Contact c : [select id,phone from Contact where id != null AND phone != null]){
 
         	if(c!=null && c.phone!=null && c.phone.length()>0){
	         	if(c.phone.contains('-')==true){
	         		c.phone = c.phone.replaceAll('-','');
	         	}
	         	
	         	if(c.phone.contains('(')==true){
					c.phone = c.phone.replaceAll('\\(','');
	         	}
				
				if(c.phone.contains(')')==true){
					c.phone = c.phone.replaceAll('\\)','');
				}
				
				if(c.phone.contains(' ')==true){
					c.phone = c.phone.replaceAll(' ','');
				}
         	}

		 //   if (map1.get(map2.get(c.phone)) == c.phone ) 
		  if (map2!=null && map2.get(c.phone) == c.phone ) 
		    { 
		    	// output matches
		        ApexPages.addMessage( new ApexPages.Message(
    			ApexPages.Severity.INFO  ,'found match:  contact phone ' +c.phone +' , external phone ' +map2.get(c.phone) +' '));
		   		i++;
		    } 
	
		} 
       
		// output count of records processed
		ApexPages.addMessage( new ApexPages.Message(
    	ApexPages.Severity.INFO  ,'matches ' +i ));
    	
		} catch(NullPointerException e){
			ApexPages.addMessages(e);
		}
	}