Can someone please help solve this NullPointer Exception:

Error:Apex trigger TruBidCompletion caused an unexpected exception, contact your administrator: TruBidCompletion: execution of BeforeUpdate caused by: System.NullPointerException: Attempt to de-reference a null object: Trigger.TruBidCompletion: line 31, column 1

trigger TruBidCompletion on Opportunity (before update)
{
public static Boolean IsWeekendDay(Date dateParam)
   {
      boolean result     = false;
      system.debug('dateParam = '+dateParam); 
      //Recover the day of the week
      Date startOfWeek   = dateParam.toStartOfWeek();
      system.debug('startOfWeek = '+startOfWeek);
      Integer dayOfWeek  = dateParam.day() - startOfWeek.day();
      system.debug('dayOfWeek = '+dayOfWeek);   
      result = dayOfWeek == 0 || dayOfWeek == 6 ? true : false;
      system.debug('result = '+result); 
      return result;
   } 
   public static Date AddBusinessDays(Date StartDate, integer BusinessDaysToAdd )
   {
      //Add or decrease in BusinessDaysToAdd days 
      Date finalDate = StartDate;
      system.debug('finaldate = '+finalDate);
      integer direction = BusinessDaysToAdd < 0 ? -1 : 1;
      system.debug('direction = '+direction);
       while(BusinessDaysToAdd != 0)
       {
           finalDate = finalDate.AddDays(direction);
           system.debug('BusinessDaysToAdd = '+BusinessDaysToAdd);            
           system.debug('finaldate = '+finalDate);
           if (!isWeekendDay(finalDate))
           {
               BusinessDaysToAdd -= direction;
               
           }
       }
       return finalDate;
   }
 {
  for(Opportunity o : Trigger.new)
    {
    /* If the user selects Round 1 as the Number of TruBid Rounds */
    if(o.Number_of_Rounds_in_TruBid__c=='Round 1')
    {
    /* Call the AddBusinessDays method called above to add 'x' days */
    o.Contract_Draft_Requested_Start_Date__c=o.Rd_1_Results_Review_End_Date__c;
    o.Contract_Draft_Requested_End_Date__c=AddBusinessDays( o.Contract_Draft_Requested_Start_Date__c,1);
    o.Award_Start_Date__c=o.Contract_Draft_Requested_End_Date__c;
    o.Award_End_Date__c=AddBusinessDays(o.Award_Start_Date__c,15);
    o.TB_Draft_Received_Start_Date__c= o.Award_End_Date__c;
    o.TB_Draft_Received_End_Date__c=o.Award_End_Date__c;
    o.TB_Edits_in_Progress_Start_Date__c=o.TB_Draft_Received_End_Date__c;
    o.TB_Edits_in_Progress_End_Date__c=AddBusinessDays(o.TB_Edits_in_Progress_Start_Date__c,47);
    o.TB_Contract_Signed_Start_Date__c=o.TB_Edits_in_Progress_End_Date__c;
    o.TB_Contract_Signed_End_Date__c=AddBusinessDays(o.TB_Contract_Signed_Start_Date__c,1);
    }