I am getting a null pointer exception.Do not know what mistake am doing 

CODE

trigger UpdateTransactRevenue on Order_Entry__c (before insert, before update) {
  Set<Id> productIdList=new Set<Id>();
  Set<Id> orderIdList=new Set<Id>();

  
  //process all records if they are new
  if(Trigger.isInsert)
  {
    for(Order_Entry__c oe:Trigger.new)
    {
      if(oe.Product__c!=null && oe.Order_Number__c!=null)
      {
        productIdList.add(oe.Product__c);
        orderIdList.add(oe.Order_Number__c);
      }
    }
  }
  
  if(Trigger.isUpdate)
  {
    for(Order_Entry__c oe:Trigger.new)
    {
         if(oe.Order_Number__c!=null && oe.Product__c!=null && oe.Product__c!=Trigger.oldMap.get(oe.Id).Product__c)
         {
         
           productIdList.add(oe.Product__c);
           orderIdList.add(oe.Order_Number__c);

           
         }
    }
  }
  
  
  
  if(productIdList.size()>0) 
  {
    //query for employee records
    Map<Id,Product_2020__c> productMap=new map<Id,Product_2020__c>([Select Id,Invoice_Rate_Blend__c , Invoice_Rate_Event__c from Product_2020__c where Id in:productIdList]);
    Map<Id,Order_2020__c>   orderMap = new Map<Id,Order_2020__c>([Select Id , Name , Transact_Kiosk_Category__c from Order_2020__c where Id in:orderIdList]);
    
    String recordtypeid=[Select Id from RecordType where Name='2020RK-VZ' and SobjectType='Order_Entry__c'].Id;
       
    if(orderMap!=null && orderMap.size()>0){
        for(Order_Entry__c oe:Trigger.new)
        {
         if(oe.RecordTypeId==recordtypeid){
           
           
               
               if(orderMap.get(oe.Order_Number__c)!=null && orderMap.get(oe.Order_Number__c).Transact_Kiosk_Category__c=='Event' && productMap!=null && productMap.get(oe.Product__c)!=null && productMap.size()>0 && productMap.get(oe.Product__c).Invoice_Rate_Event__c!=null)
               {
                   oe.Transact_Revenue__c=productMap.get(oe.Product__c).Invoice_Rate_Event__c * oe.Quantity__c; 
               }
               else if(orderMap.get(oe.Order_Number__c)!=null && orderMap.get(oe.Order_Number__c).Transact_Kiosk_Category__c!='Event'&& productMap!=null && productMap.get(oe.Product__c)!=null && productMap.size()>0 && productMap.get(oe.Product__c).Invoice_Rate_Blend__c!=null){
               
                   oe.Transact_Revenue__c=productMap.get(oe.Product__c).Invoice_Rate_Blend__c * oe.Quantity__c; 
               }
           
          }       
        } 
    }       
  } 

ERROR

caused by: System.NullPointerException: Argument 1 cannot be null

Trigger.UpdateTransactRevenue: line 57, column 1(highlighted is line 57)

Can any body help me please