Null pointer exception on bulk update but working when I update manually via UI

Question

I'm running a batch update on tasks and my task trigger handler is throwing a null pointer exception error. If I go through and update the tasks individually, I don't have any issues with them. If I run a batch of 2000, 1600 will update and 400 will throw the error. I'm completely at a loss why this is happening.

The following line is where I'm getting the null pointer errors:

if(String.ValueOf(t.WhatId).left(3) == '003'){

Complete code:

public class TaskTriggerHandler {
    String stdRecTypeId = [SELECT id from RecordType WHERE sObjectType = 'Task' AND Name = 'Standard'].id;

    //execute after insert/after update
    public void isAfterInsertTask(Map<Id, Task> newTaskTriggerMap){
        Set<Id> acctIds = new Set<Id>();
        Set<Id> opptyIds = new Set<Id>();

        Map<Id, Id> contactIdAcctIdMap = new Map<Id, Id>();
        List<Id> contactIds = new List<Id>();
        for(Task t : newTaskTriggerMap.values()){
            if(t.whoId != NULL){
                contactIds.add(t.whoId);
            }
        }
        for(Contact c : [SELECT Id, AccountId FROM Contact WHERE id IN :contactIds]){
            contactIdAcctIdMap.put(c.Id, c.AccountId);
        }
        //loop through tasks and put account/oppty id's into sets
        for(Task t : newTaskTriggerMap.values()){
            if(t.WhatId == NULL && String.valueOf(t.WhoId).left(3) == '003'){
                acctIds.add(contactIdAcctIdMap.get(t.whoId));
            } else if(t.whatId != NULL && String.valueOf(t.WhatId).left(3) == '001'){
                acctIds.add(t.AccountId);
            } else if(t.whatId != NULL && String.valueof(t.WhatId).left(3) == '006'){
                opptyIds.add(t.whatId);
            }
        }
        List<Account> acctsToUpdate = new List<Account>();
        //execute account update logic
        if(!acctIds.isEmpty()){
            List<Account> accts = [SELECT id, Number_of_Scheduled_Activities__c,
                                   Number_of_Completed_Activities__c, Next_Activity_Date__c,
                                   Last_Completed_Activity_Date__c, Next_Activity_Assigned_To__c
                                   FROM Account WHERE Id IN :acctIds];
            List<Task> acctTasks = [SELECT id, ActivityDate, OwnerId, IsClosed, AccountId, WhatId, RecordTypeId
                                    FROM Task WHERE AccountId = :acctIds];
            List<Opportunity> acctOpptys = [SELECT id, AccountId, IsClosed, CloseDate 
                                            FROM Opportunity WHERE AccountId IN :acctIds AND IsClosed = False 
                                            ORDER BY CloseDate ASC];
            Map<Id, List<Task>> mapAcctidsTasks = new Map<Id, List<Task>>();
            for(Task t : acctTasks){
                if(!mapAcctIdsTasks.containsKey(t.AccountId)){
                    mapAcctIdsTasks.put(t.AccountId, new List<Task>());
                }
                mapAcctIdsTasks.get(t.AccountId).add(t);
            }

            Map<Id, List<Opportunity>> mapAcctIdOpptys = new Map<Id, List<Opportunity>>();
            if(!acctOpptys.isEmpty()){
                for(Opportunity o : acctOpptys){
                    if(!mapAcctIdOpptys.containsKey(o.AccountId)){
                        mapAcctIdOpptys.put(o.AccountId, new List<Opportunity>());
                    }
                    mapAcctIdOpptys.get(o.AccountId).add(o);
                }
            }
            List<Task> tasksToUpdate = new List<Task>();
            //loop through accounts and increment task counters tasks 
            for(Account a : accts){
                Decimal numOpenTasks = 0;
                Decimal numClosedTasks = 0;
                Date nextScheduled; 
                Date lastCompleted = a.Last_Completed_Activity_Date__c;
                Id nextAssigned = a.Next_Activity_Assigned_To__c;
                if(!mapAcctIdOpptys.isEmpty()){
                    for(Task t : mapAcctIdsTasks.get(a.id)){
                        Task updatedTask = new Task(id=t.id);
//The following line is where I'm getting the null pointer errors

                        if(String.ValueOf(t.WhatId).left(3) == '003'){

                               updatedTask.whatId = mapAcctIdOpptys.get(t.WhatId)[0].Id;
                               tasksToUpdate.add(updatedTask);
                           }
                        if(!t.IsClosed){
                            numOpenTasks++;
                            if((nextScheduled == NULL || t.ActivityDate <= nextScheduled) && 
                               t.ActivityDate >= date.today()){
                                   nextAssigned = t.OwnerId;
                                   nextScheduled = t.ActivityDate;
                               }
                        }
                        if (t.IsClosed){
                            numClosedTasks++;
                            if(lastCompleted == NULL || t.ActivityDate >= lastCompleted){
                                lastCompleted = t.ActivityDate;
                            }
                        }
                    }
                }
                Account acctToUpdate = new Account(Id = a.id);
                Boolean wasUpdated = false;
                if(nextAssigned != a.Next_Activity_Assigned_To__c){
                    acctToUpdate.Next_Activity_Assigned_To__c = nextAssigned;
                    wasUpdated = true;
                }
                if(nextScheduled != a.Next_Activity_Date__c){
                    acctToUpdate.Next_Activity_Date__c = nextScheduled;
                    wasUpdated = true;
                }
                if(lastCompleted != a.Last_Completed_Activity_Date__c){
                    acctToUpdate.Last_Completed_Activity_Date__c = lastCompleted;
                    wasUpdated = true;
                }
                if(numClosedTasks != a.Number_of_Completed_Activities__c){
                    acctToUpdate.Number_of_Completed_Activities__c = numClosedTasks;
                    wasUpdated = true;
                }
                if(numOpenTasks != a.Number_of_Scheduled_Activities__c){
                    acctToUpdate.Number_of_Scheduled_Activities__c = numOpenTasks;
                    wasUpdated = true;
                }
                if(wasUpdated){
                    acctsToUpdate.add(acctToUpdate);
                }
            }
            if(!acctsToUpdate.isEmpty()){
                update acctsToUpdate;
            }
            if(!tasksToUpdate.isEmpty()){
                update tasksToUpdate;
            }
        }

Answer 1

Just add a guard clause. You should never enforce that a field is not null by just assuming it will always be populated.

Instead use:

if (t.WhatId != null && t.WhatId.getSObjectType() == Contact.sObjectType)