I have a fix, but anyone have any ideas why this is happening?

Is my solution the best way to address error?

I have the following trigger giving me an "System.NullPointerException: Attempt to de-reference a null object" error intermittent error on  line 28 - newAttendee.Contact__c.addError('This contact is already registered for this event.');

Using test data in a sandbox the trigger works on some and throw exception on others.

Here is an overview of custom objects:

Event

- Registration

- Lookup from Registration to Event

- - Attendee

- - Master detail between Registration and Attendee

- - EventDetail__c is a formula  {eventID-contactID}

Here is the trigger

trigger TRG_Attendee_singleContactPerEvent on Attendee__c (before insert, before update) {
    // Make sure we are not adding a Attendee to the same Event     
        
    Map<String, Attendee__c> attendeeMap = new Map<String, Attendee__c>();
    for (Attendee__C Attendee : System.Trigger.new) {
        
        // Only check if adding to an active Registration.  
        if (Attendee.Registration__r.Status__c  != 'Canceled')
            {
            // Make sure another new Attendee in batch isn't also a duplicate  
            if (attendeeMap.containsKey(Attendee.EventContact__c)) {
                // If another new Attendee in batch is a duplicate set error  
                Attendee.Contact__c.addError('This contact is already registered for this event.');
            } else {
                // Add to list for check
                attendeeMap.put(Attendee.EventContact__c, Attendee);
            }
        }
    }
    

    // Using a single database query, find all the Attendees in  
    // the database that have the same EventContact as any  
    // of the Attendees being inserted or updated.  
    for (Attendee__C foundAttendee : [SELECT EventContact__c FROM Attendee__c
                      WHERE EventContact__c IN :attendeeMap.KeySet() and Registration_Status__c = 'Active']) {
            Attendee__C newAttendee = attendeeMap.get(foundAttendee.EventContact__c);
            newAttendee.Contact__c.addError('This contact is already registered for this event.');
    }
}

I have fixed by putting if statement around line 27 and 28 like:

        if (attendeeMap.containsKey(foundAttendee.EventContact__c)){
            Attendee__C newAttendee = attendeeMap.get(foundAttendee.EventContact__c);
            newAttendee.Contact__c.addError('This contact is already registered for this event.');
        }