[SalesForce] Attempt to de-reference a null object Error is in expression ‘{!ReadFile}’ in component in page uploadaccountsall

Hi i am trying to check whether the account name exists or not, while uploading a csv file to create account, contact and opportunity. But in this i am getting error,i tried a lot but i can't resolve this. my code apex class look like ths.

List<String> acctNames;
public Pagereference ReadFile(){
nameFile=contentFile.toString();
filelines = nameFile.split('\n');
accstoupload = new List<Account>();
contoupload = new List<Contact>();
opptoupload = new List<Opportunity>();
custtoupload = new List<CustomLead__c>();

 for (Integer i=1;i<filelines.size();i++){
     String[] inputvalues = new String[]{};
     inputvalues = filelines[i].split(',');
     Account a = new Account();
     a.Name = inputvalues[0];
     acctNames.add(a.Name);
     accstoupload.add(a);
  }
     List<Account> existingAccts = [SELECT Id, Name FROM Account where name in  :acctNames];
   //create a map with names as key
   Map<String, Id> acctNamesIdMap = new Map<String, Id>();
   // load the map - this will help you find out if an account name exists already
   for (Account acct : existingAccts){
    acctNamesIdMap.put(acct.Name, acct.Id);
    System.debug('******Sai******');
  }
   List<Account> newAccts = new List<Account>();
  for (Account acct : accstoupload){
    //if account name does not exist in map, add it to list of new accounts
    if (!acctNamesIdMap.containsKey(acct.Name)){
     newAccts.add(acct);
   }
  }
   try{
 //insert accstoupload;
  insert newAccts;
  }
   catch (Exception e){
    ApexPages.Message errormsg = new ApexPages.Message(ApexPages.severity.ERROR,'An   error has occured. Please check the template or try again later');
   ApexPages.addMessage(errormsg);
   } 

public List<Account> getuploadedAccounts(){

  if (accstoupload!= NULL)
    if (accstoupload.size() > 0)
       return accstoupload;
       else
       return null;
     else
     return null;

Error Mg:
Attempt to de-reference a null object
Error is in expression '{!ReadFile}' in component in page uploadaccountsall