Test Class for Scheduled Batch Class – Error

I have a scheduled batch class that will eventually run on the 1st of every month. It's designed to find records from an Asset that are linked to Accounts, aggregate the counts of these records into 3 categories (New, Old, Active), and then create a single custom object (Track__c) record for each associated account.

I created the test class below as a start (it still needs Assertions) but I'm getting an error message:

System.ListException: Row with null Id at index: 0 ||
Class.MembChurnRecordsBatch.execute: line 16, column 1

I'm don't know how to address this, can anyone advise?

Test Class:

@istest
public class RecordsTest{
    
    public static testmethod void testBatch1() {
        Test.startTest();
        
        Account account = TestUtility.createAccount('Prospect', RECORDTYPEID_ACCOUNT_TYPE);
        insert account;
        
        Contact contact = TestUtility.createContact(account.Id, RECORDTYPEID_CONTACT_TYPE, 'Primary');
        insert contact;
        
        Asset rec = new Asset(AccountId=account.Id,Name='Test',StartDate=System.today().addMonths(-3),EndDate=System.Today());
        insert rec;
        
        BatchSchedule__c batchSchedRec = new BatchSchedule__c();
        insert batchSchedRec;

         string chron = '0 0 23 * * ?';
            string jobid = System.schedule('testScheduledApex', chron, new RecordsSchedule());
            CronTrigger ct = [Select id , CronExpression from CronTrigger where id = :jobId];

        Tracker__c track = new Tracker__c();
            track.Account_Name__c=account.Id;
            track.Active__c=0;
            track.New__c=0;
            track.Old__c=0;
            track.Date__c=System.today();
        insert track;

        batchSchedRec = [SELECT Id,Scheduled_Id__c FROM BatchSchedule__c WHERE ID =: batchSchedRec.Id];
            batchSchedRec.Scheduled_Id__c = ct.Id;
        update batchSchedRec;
        
        RecordsBatch testBatch = new RecordsBatch();

        Database.executebatch(testBatch);      
        Test.stopTest();
    }
    
    
    public static testmethod void testSchedule() {
        Test.startTest();
        
        BatchSchedule__c batchSchedRec = new BatchSchedule__c();
            batchSchedRec.Scheduled_Id__c = '0';
        insert batchSchedRec;
        
        RecordsSchedule testSched = new RecordsSchedule();
        String sch = '0 0 1 1 * ? *';
        system.schedule('Test status Check',sch,testSched);
        
        Test.stopTest();
        
    }
}

Class:

public class RecordsBatch implements Database.Batchable<sObject> {
    public Database.QueryLocator start(Database.BatchableContext context) {
        return Database.getQueryLocator([SELECT Id FROM Account WHERE Id IN (SELECT AccountId FROM Asset)]);
    }

Date PriorMonth = System.today().toStartOfMonth().addMonths(-1);
Date CurrentMonth  = System.today().toStartOfMonth();
Integer OldAmt;
Integer NewAmt;
Integer ActiveAmt;

    public void execute(Database.batchableContext context, List<Account> scope) {

        List<Tracker__c> RecList = new List<Tracker__c>();

        Map<Id, AggregateResult> activeRec = new Map<Id, AggregateResult>([
            SELECT AccountId Id1, COUNT(Id) amt1, EndDate
            FROM Asset 
            WHERE AccountId = :scope AND AccountId != Null AND EndDate >=:CurrentMonth AND EndDate <> NULL GROUP BY AccountId, EndDate]);

        Map<Id, AggregateResult> newRec = new Map<Id, AggregateResult>([
            SELECT AccountId Id2, COUNT(Id) amt2, StartDate, EndDate
            FROM Asset 
            WHERE AccountId = :scope AND AccountId != Null AND StartDate >= :PriorMonth AND StartDate <> NULL AND StartDate < :CurrentMonth GROUP BY AccountId, StartDate, EndDate]);            

        Map<Id, AggregateResult> oldRec = new Map<Id, AggregateResult>([
            SELECT AccountId Id3, COUNT(Id) amt3, EndDate
            FROM Asset 
            WHERE AccountId = :scope AND AccountId != Null AND EndDate >= :PriorMonth AND EndDate <> NULL AND EndDate < :CurrentMonth GROUP BY AccountId, EndDate]);

        for(Account record: scope) {
            AggregateResult activeCount = activeRec.get(record.Id);
                if(activeCount != null) {
                    ActiveAmt = (Integer)activeCount.get('amt1');
                } else {
                    ActiveAmt = null;
                }
            AggregateResult newCount = newRec.get(record.Id);
                if(newCount != null) {
                    NewAmt = (Integer)newCount.get('amt2');
                } else {
                    NewAmt = null;
                }
            AggregateResult oldCount = oldRec.get(record.Id);
                if(oldCount != null) {
                    OldAmt = (Integer)oldCount.get('amt3');
                } else {
                    OldAmt = null;
                }
            Tracker__c ct = new Tracker__c(Account_Name__c = record.Id, Date__c = PriorMonth, Active__c = ActiveAmt, New__c = NewAmt, Old__c = OldAmt);
                RecList.add(ct);
            }
        insert RecList;
    }

    public void finish(Database.BatchableContext bc)
    {
        BatchSchedule__c b = BatchSchedule__c.getOrgDefaults();
            b.Scheduled_Id__c = system.scheduleBatch(new RecordsBatch(),'Batch 2'+System.currentTimeMillis(),2);
        upsert b;
    }
}