Good Morning,

I'm not a developer but I put together a trigger to update a checkbox field (Trigger_Email_on_Outstanding_Task__c) on the related Lead Object if the corresponding field on a Task (Trigger_email_on_Outstanding_Task__c) is checked (by a workflow):
____________________________________________________________________________________
trigger TriggerTaskNotification on Task (after insert, after update) {


    Set<Id> leadIds=new Set<Id>();

    for(Task t:trigger.new){
        if(t.Trigger_Email_on_Outstanding_Task__c==true){
            if(String.valueOf(t.whoId).startsWith('00Q')==TRUE){//check if the task is associated with a lead
              
              if( !leadIds.contains(t.whoid)){   //Dont forget '!'           
                                 leadIds.add(t.whoId);
                      }//if3
               
            }//if 2
        }//if 1
    }//for
   
    /* List<Lead> leadsToUpdate=[SELECT Id, Trigger_Email_on_Outstanding_Task__c FROM Lead WHERE Id IN :leadIds AND Trigger_Email_on_Outstanding_Task__c= True]; */

   Map<Id,Lead>  relatedleads = new Map<Id,Lead>([SELECT Id, Trigger_Email_on_Outstanding_Task__c FROM Lead WHERE Id IN :leadIds]);


   For(Task t2 : trigger.new){
            if(t2.Trigger_Email_on_Outstanding_Task__c==true){

                       if(relatedleads.get(t2.whoid) != null){  //if task is related to lead

                                     //make lead field to 'checked'
                                     relatedleads.get(t2.whoid).Trigger_Email_on_Outstanding_Task__c = true;
                        }//if2

           }//if1

}//for
 
    try{
        update relatedleads.values();
    }catch(DMLException e){
        system.debug('Leads were not all properly updated.  Error: '+e);
    }
}

______________________________________________________________________________

Looks like i'm receiving a System.NullPointerException: Attempt to de-reference a null object error, on execution of AfterUpdate
from line 8, column 1
if(String.valueOf(t.whoId).startsWith('00Q')==TRUE){//check if the task is associated with a lead

I'm assuming that it may be due to a task that doesn't have a related lead record but where the workflow has checked the checkbox on the task.  Is there a line I can omit or add that can account for these instances, where if there's a task with the checkbox checked but with no associated whoid then ignore....i'm assuming my trigger isn't very cleanly written since it's written by me. 

thanks everyone