System.DmlException: Delete failed. CANNOT_INSERT_UPDATE_ACTIVATE_ENTITY, execution of BeforeDelete

Question

BACKGROUND

I have a simple trigger which is meant to block a record from being deleted based on the record type.

I am using a Trigger Handler by Kevin O'Hara

Trigger

trigger InvoiceTrigger on Invoice__c (before delete) {
    new InvoiceTriggerHandler().run();    
}

Trigger Handler

public class InvoiceTriggerHandler extends TriggerHandler {

    public InvoiceTriggerHandler() {}

    public override void beforeDelete(){

        for(Invoice__c invoice : (List<Invoice__c>) Trigger.new) {

            if (IsLocked(invoice)){
                invoice.addError('You cannot delete an Invoice record which is locked. Please contact your Salesforce Administrator.');
            }
        }   
    }

    private boolean IsLocked(Invoice__c invoice){
        return invoice.RecordTypeId == InvoiceUtils.getLockedRecordTypeId;
    }
}

Trigger Unit Test

@isTest
public class InvoiceTriggerTests {

    @isTest
    public static void InvoiceTrigger_Success(){

        // arrange
        Id invoiceId = TestDataFactory.createInvoice();

        // act
        Database.delete(invoiceId);

        // assert
        System.assert(true); // don't judge me :)
    }
}

But when I run the unit test I get this error message:

Error Message

System.DmlException: Delete failed. First exception on row 0; first error: CANNOT_INSERT_UPDATE_ACTIVATE_ENTITY, InvoiceTrigger: execution of BeforeDelete

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

Class.InvoiceTriggerHandler.beforeDelete: line 7, column 1 Class.TriggerHandler.run: line 43, column 1 Trigger.InvoiceTrigger: line 2, column 1: []

QUESTIONS

1. What is actually causing the problem?
2. How can I fix it?

Answer 1

In the delete context, your trigger.new will be null. Use trigger.old instead for delete events.