I've got a Batchable Apex test I've created that is throwing the following error:

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

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

Trigger.setTrainingAccountId: line 55, column 1: []

I'm not sure why the Trigger is affecting my Test Class Execution. I haven't been able to pinpoint the exact issue. Any ideas?

Trigger

trigger setTrainingAccountId on Training__c (before insert, before update, after insert, after update, after delete, after undelete) {
    set<Id> setAttendees = new set<Id>();
    set<Id> setCourses = new set<Id>();

  set<Id> setUnknownAccounts = new set<Id>();
  
  for(Account a : [
    SELECT
      Id
    FROM
      Account
    WHERE
      Name LIKE '%End%User%'
      OR Name LIKe '%User%End'
  ]){
    setUnknownAccounts.add(a.Id);
  }
    
    if(trigger.isDelete){
        for(Training__c t : trigger.old){
            setAttendees.add(t.Attendee__c);
            setCourses.add(t.Course_or_Module__c);
        }
    }else{
        for(Training__c t : trigger.new){
            setAttendees.add(t.Attendee__c);
            setCourses.add(t.Course_or_Module__c);
        }
    }
    
    map<Id,Contact> mapContactAccount = new map<Id,Contact>([
    SELECT
            Id,
            AccountId
        FROM
            Contact
        WHERE
            Id IN : setAttendees
    ]);
    
    //map<Id, Course__c> mapCourses = new map<Id, Course__c>([SELECT Name, Duration_Hours__c FROM Course__c WHERE Id IN : setCourses]);
    map<Id,Course__c> mapCourses = new map<Id,Course__c>([
        SELECT
            Id,
            Name
        FROM
            Course__c
        WHERE
            Id IN : setCourses
    ]);
    
    //Jade Update : Account field update with current contact's account
    if(trigger.isBefore){
        for(Training__c t : trigger.new){
          if(setUnknownAccounts.contains(mapContactAccount.get(t.Attendee__c).AccountId)){
          t.addError('Assigning Training to a Contact associated with an Unknown End User Account is not allowed. Pleae assign the Contact to an existing Account or create a new one first.');
        }
            t.Account__c = mapContactAccount.get(t.Attendee__c).AccountId;
        }
    }
    /* if(trigger.isBefore){
        for(Training__c t : trigger.new){
            t.Account__c = mapContactAccount.get(t.Attendee__c).AccountId;
            if(t.Duration_Hours__c == null && trigger.isInsert){
                t.Duration_Hours__c = mapCourses.get(t.Course_or_Module__c).Duration_Hours__c;
            }
        }
    } */
    
    if(trigger.isAfter){
        set<Id> setAccounts = new set<Id>();
        
        if(trigger.isDelete){
            for(Training__c t : trigger.old){
                if(t.Course_or_Module__c != null){
                    if(
                        mapCourses.get(t.Course_or_Module__c).Name.startsWithIgnoreCase('pivCLASS Advanced Training')
                        || mapCourses.get(t.Course_or_Module__c).Name.startsWithIgnoreCase('pivCLASS PKI @ the Door Training')
                        || mapCourses.get(t.Course_or_Module__c).Name.startsWithIgnoreCase('pivCLASS Software Training')
                    ){
                        setAccounts.add(mapContactAccount.get(t.Attendee__c).AccountId);
                    }
                }
            }
        }else{
            for(Training__c t : trigger.new){
                if(t.Course_or_Module__c != null){
                    if(
                        mapCourses.get(t.Course_or_Module__c).Name.startsWithIgnoreCase('pivCLASS Advanced Training')
                        || mapCourses.get(t.Course_or_Module__c).Name.startsWithIgnoreCase('pivCLASS PKI @ the Door Training')
                        || mapCourses.get(t.Course_or_Module__c).Name.startsWithIgnoreCase('pivCLASS Software Training')
                    ){
                        setAccounts.add(mapContactAccount.get(t.Attendee__c).AccountId);
                    }
                }
            }
        }
        
        if(!setAccounts.isEmpty()){
            accountCertifications accountCertifications = new accountCertifications();
            accountCertifications.setAccountCertifications(setAccounts);
        }
    }
}

Test Class

@isTest
public class TestBatchUpdateTraining {

    Static testmethod void test(){
String Query = 'SELECT id,Training_Status__c,Expiry_Date__c FROM Training__c WHERE Expiry_Date__c < Today';
     

    
Training__c[] ml = new List<Training__c>();
        date mydate = date.parse('5/11/2014');
       for (Integer i=0;i<10;i++) {
           Training__c m = new Training__c(
               Course_or_Module__c= 'a0vn0000000EZsJ',
               Account__c = '001n0000005m8G0',
               Attendee__c ='003n0000006QMkP', //Learner
               Training_Status__c='Completed',
           		Expiry_Date__c = mydate);
           ml.add(m);
           System.debug(ml);
       }
    insert ml;
    
    Test.startTest();
    BatchUpdateTraining bit = new BatchUpdateTraining();
    Database.executeBatch(bit);
    Test.stopTest();
    
    Integer i = [SELECT COUNT() FROM Training__c WHERE Training_Status__c = 'Expired'];
    System.assertEquals(i, 0);
    }
}

Batch Update

global class BatchUpdateTraining implements Database.Batchable<sobject> {

     public String Query;

     public Database.QueryLocator start(Database.BatchableContext BC)  {  

        Query = 'SELECT id,Training_Status__c,Expiry_Date__c FROM Training__c WHERE Expiry_Date__c < Today';
		
        return Database.getQueryLocator(Query); 

    }

     public void execute(Database.BatchableContext BC, List<Training__c> scope) { 
        List<Training__c> tr = new List<Training__c>(); 
		System.debug(scope);
        for (Training__c ac : scope)  {
		ac.Training_Status__c = 'Expired';
        tr.add(ac);
        System.debug(tr.size());

       }

           update tr;
     }

 
    public void finish(Database.BatchableContext BC) {   
    AsyncApexJob a = [SELECT Id, Status, NumberOfErrors, JobItemsProcessed,
      TotalJobItems, CreatedDate, CreatedBy.Email
      FROM AsyncApexJob WHERE Id =
      :BC.getJobId()];
	Messaging.SingleEmailMessage mail = new Messaging.SingleEmailMessage();
    List<String> toAddresses = new List<String> {a.CreatedBy.Email};
        mail.setToAddresses(toAddresses);
        mail.setSubject('Training Expiration Job Processed ' + a.Status);
        mail.setPlainTextBody('The batch Apex job processed ' + a.TotalJobItems + ' batches with ' + a.NumberOfErrors + ' failures' + a.CreatedDate + 'Date Ran');
        Messaging.sendEmail(new Messaging.SingleEmailMessage[] {mail});
   }

}