Please help, I have 50% coverage for the trigger below. However, I am receiving the following error and nor do I know if I'm writing the correct test class: 

System.DmlException: Insert failed. First exception on row 0; first error: CANNOT_INSERT_UPDATE_ACTIVATE_ENTITY, RollupEnrollmentCount: execution of AfterInsert
caused by: System.NullPointerException: Attempt to de-reference a null object
Trigger.RollupEnrollmentCount: line 43, column 1: []
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trigger RollupEnrollmentCount on DJEnrollment__c (after insert, after update, after delete) {

     Set<Id> CampaignIds = new Set<Id>();
   
   if(Trigger.isDelete) {
     for(DJEnrollment__c test:Trigger.Old) {
      
        CampaignIds.add(test.EnrollClass_Name__c);   
    
    }   
   
   }
   else
   if(Trigger.isUpdate) {

     for(DJEnrollment__c test:Trigger.New) {
      
        CampaignIds.add(test.EnrollClass_Name__c);   
    
     }

     for(DJEnrollment__c test:Trigger.Old) {
      
        CampaignIds.add(test.EnrollClass_Name__c);   
    
     }   
   
   }
   else
   {
     for(DJEnrollment__c test:Trigger.New) {
      
        CampaignIds.add(test.EnrollClass_Name__c);   
    
     }
   }

    AggregateResult[] groupedResults = [SELECT count(id)enrollmentCount, EnrollClass_Name__c FROM DJEnrollment__c WHERE EnrollClass_Name__c in :CampaignIds GROUP BY EnrollClass_Name__c];
    
    Map<id,Campaign> campaignMap = new Map<Id,Campaign>([SELECT id, Class_Total_Enrollments__c FROM Campaign WHERE Id in :CampaignIds]); 
    
    for(AggregateResult ar: groupedResults) {
          campaignMap.get((Id)ar.get('EnrollClass_Name__c')).Class_Total_Enrollments__c = (decimal) ar.get('enrollmentcount');
    }
    
    try {
        update campaignMap.values();
    }   catch(DmlException e) {
        System.debug(e.getMessage());
    }

}