I have Bigmachine Custom Object

In this i have 3 Fields

1. Scope__c

2. Scope1__c

3. Scope2__c.

Scope1__c and Scope2__c are Formula fields and Scope__c field is Number and is summation of Scope1__c and Scope2__c.

Below trigger gives me proper Scope__c values.

Trigger generateFields on Bigmachine__c (before insert,before update) {       

Bigmachine__c big = Trigger.New[0];            

big.Scope__c=big.Scope1__c-der.Scope2__c;

}

When I am writing Test class for this Trigger I am getting this :

System.DmlException: Insert failed. First exception on row 0; first error: CANNOT_INSERT_UPDATE_ACTIVATE_ENTITY, generateFields: execution of BeforeInsert

caused by: System.NullPointerException: Attempt to de-reference a null object

Trigger.generateFields: line 3, column 1: []

My Test class is:

@isTest(SeeAllData=true)
public with sharing class generateFieldsTest { 
 
    static testMethod void validatefields(){ 

    Bigmachine__c big = new Bigmachine__c();

    list<Bigmachine__c> bg = [select  Scope1__c, Scope2__c from Bigmachine__c];
    for(Bigmachine__c b : bg){
      if(b.Scope1__c !=null && b.Scope2__c != null){
      big.Scope__c = b.Scope1__c - b.Scope2__c;
      if(big.Scope__c > 0){
        try{
            insert big;
             }catch(dmlexception de){
            system.debug('!!!!!!' +de.getmessage());
            throw de;
            }
          }
        }
     }
   }
}

I am getting somehow code coverage but my test method is getting failed.

Can somebody please help me out to pass my test class and get the proper code coverage

Thanks in advance!!!!!!!!