Need some help with a Trigger Error. I am not a developer so my skills get stretched with things like this. Also, for whatever it's worth I did not write the Trigger but am in a position to support it. The user is getting the following error when they try and merge records: "AccountTrigger: execution of AfterDelete caused by: System.NullPointerException: Attempt to de-reference a null object: External entry point"."

See below Trigger syntax:

trigger AccountTrigger on Account (before insert, after insert, after update, after delete) {
    String PROSPECT_RT_ID = dao_Account.getInstance().getRecordTypeId('Prospect');
    User defaultOwner = dao_User.getInstance().getUserByName('Medical-Surgical IT Data');
    List<AccountTeamMember> atmInserts = new List<AccountTeamMember>();
    List<AccountShare> ashInserts = new List<AccountShare>();
    String LTC_REP = 'EC Long Term Care Rep';
    String HC_REP = 'EC Home Care DME Rep';     

    Map<Id, Searchable_Account__c> saMap = new Map<Id, Searchable_Account__c>();    
    for (Searchable_Account__c sa : [Select Id, Account__c, Account_Name_ExtId__c from Searchable_Account__c where Account__c in :trigger.new]){
        saMap.put(sa.Account__c, sa);
    }
    for (Account a : Trigger.new)
    {
        if (a.RecordTypeId == PROSPECT_RT_ID)
        {
            if (trigger.isBefore)
            {
                a.OwnerId = defaultOwner.Id;
            } else if (!trigger.isDelete){              
                User originalOwner = dao_User.getInstance().getUser(a.CreatedById);
                AccountTeamMember atm = new AccountTeamMember();
                AccountShare ash = new AccountShare();
                atm.AccountId = a.Id;
                atm.UserId = originalOwner.Id;
                if ( originalOwner.Profile.Name.contains('HomeCare') || (originalOwner.UserRoleId != null && originalOwner.UserRole.Name.startsWith('HC'))  )
                {
                    atm.TeamMemberRole = HC_REP;
                } else if ( originalOwner.Profile.Name.contains('LongTermCare')  || (originalOwner.UserRoleId != null && originalOwner.UserRole.Name.startsWith('LTC')) ) {
                    atm.TeamMemberRole = LTC_REP;
                }
                atmInserts.add(atm);
                
                ash.AccountAccessLevel = 'Edit';
                ash.AccountId = a.Id;
                ash.OpportunityAccessLevel = 'None';
                ash.UserOrGroupId = originalOwner.Id;
                ashInserts.add(ash);
                
            }
        } else if (trigger.isAfter) {           
            Searchable_Account__c sa = saMap.get(a.id);
            if (sa== null) sa = new Searchable_Account__c();
            sa.Account__c = a.Id;
            sa.Account_Name_ExtId__c = a.Name + ' | ' + a.AccountNumber;
            saMap.put(a.Id,sa);
        }                   
    }
    insert atmInserts;
    insert ashInserts;
    upsert saMap.values();
}

I understand what is happening, and to an extent why but I am at a loss as to how to fix it. Thanks in advance for any guidance.