trigger HandoffToISRLead on Task (after insert) {

Map<Id,Id> LeadsToUpdate = new Map<Id,Id>();


for (Task t: Trigger.new) {
    if(t.Subject.startsWith('Handoff to ISR') && t.WhoId != NULL && String.valueOf(t.WhoId).startsWith('00Q'))
    
    LeadsToUpdate.put(t.WhoId, NULL);
    }

List<Lead> LeadsToHandoff = [SELECT Id, Last_ISR_ID__c, ISR_Handoff_Counter__c, LeadSource FROM Lead where Id IN:LeadsToUpdate.keyset()];

if (LeadsToUpdate.keyset().size()>0){

for (Lead l: LeadsToHandoff) {
    l.SD_Pipeline__c = true;
    if(l.ISR_Handoff_Counter__c != NULL){
    l.ISR_Handoff_Counter__c = l.ISR_Handoff_Counter__c + 1;}
    else{l.ISR_Handoff_Counter__c = 1;}

    if(l.Last_ISR_ID__c != NULL){
    l.OwnerID = l.Last_ISR_ID__c;
    }
    else if(l.LeadSource == 'Partner/Vendor'){l.OwnerID = '00530000009b58p';}
    else{l.OwnerID = '00530000009sb4g';
}
    }
    update LeadsToHandoff;
}
}

Here is some of the recent debug code:

16:16:53.139 (139810822)|CODE_UNIT_STARTED|[EXTERNAL]|01q30000000a8Hi|HandoffToISRLead on Task trigger event AfterInsert for [00T3000002gcWfr, 00T3000002gcWfs]
16:16:53.139 (139836969)|HEAP_ALLOCATE|[EXTERNAL]|Bytes:12
16:16:53.139 (139876331)|HEAP_ALLOCATE|[EXTERNAL]|Bytes:12
16:16:53.139 (139893603)|VARIABLE_SCOPE_BEGIN|[1]|this|HandoffToISRLead|true|false
16:16:53.139 (139954648)|VARIABLE_ASSIGNMENT|[1]|this|{}|0x7e3d9e08
16:16:53.139 (139977350)|HEAP_ALLOCATE|[EXTERNAL]|Bytes:12
16:16:53.139 (139987725)|VARIABLE_SCOPE_BEGIN|[1]|this|HandoffToISRLead|true|false
16:16:53.140 (140005443)|VARIABLE_ASSIGNMENT|[1]|this|{}|0x7e3d9e08
16:16:53.140 (140012249)|STATEMENT_EXECUTE|[1]
16:16:53.140 (140015277)|STATEMENT_EXECUTE|[3]
16:16:53.140 (140025872)|HEAP_ALLOCATE|[3]|Bytes:4
16:16:53.140 (140061801)|VARIABLE_ASSIGNMENT|[3]|this.LeadsToUpdate|{"serId":1,"value":{}}|0x7e3d9e08
16:16:53.140 (140138730)|HEAP_ALLOCATE|[6]|Bytes:5
16:16:53.140 (140167629)|VARIABLE_SCOPE_BEGIN|[6]|t|Task|true|false
16:16:53.140 (140589412)|VARIABLE_ASSIGNMENT|[6]|t|{"RecordTypeId":"01230000001dnNMAAY","Event_Completed__c":false,"CurrencyIsoCode":"USD","LastModifiedById":"00530000009beV7AAI","OwnerId":"00530000009beV7AAI","LastModifiedDate":"2014-05-14T23:16:53.000Z","ActivityDate":"2014-05-14T00:00:00.000Z","IsRecurrence":false,"Priority":"Normal","IsArchived":false,"Status":"Not Started","IsClosed":false,"IsVisibleInSelfServi (2 more) ...":false,"qbdialer__SMSReminde (4 more) ...":false,"DB_Activity_Type__c":"Other","IsReminderSet":false,"SystemModstamp":"2014-05-14T23:16:53.000Z","CreatedById":"00530000009beV7AAI","CreatedDate":"2014-05-14T23:16:53.000Z","IsDeleted":false,"Id":"00T3000002gcWfrEAE","IS_Response__c":false}|0x589b05d7
16:16:53.140 (140602741)|STATEMENT_EXECUTE|[6]
16:16:53.140 (140623278)|HEAP_ALLOCATE|[7]|Bytes:14
16:16:53.140 (140738987)|HEAP_ALLOCATE|[7]|Bytes:41
16:16:53.140 (140899452)|FATAL_ERROR|System.NullPointerException: Attempt to de-reference a null object

Trigger.HandoffToISRLead: line 7, column 1
16:16:53.140 (140913883)|FATAL_ERROR|System.NullPointerException: Attempt to de-reference a null object

Trigger.HandoffToISRLead: line 7, column 1
16:16:53.140 (140989867)|CODE_UNIT_FINISHED|HandoffToISRLead on Task trigger event AfterInsert for [00T3000002gcWfr, 00T3000002gcWfs]
16:16:53.140 (140997965)|EXECUTION_FINISHED

I tried looking for the 2 tasks in the debug code, but received this error when trying to access them:

Data Not Available
The data you were trying to access could not be found. It may be due to another user deleting the data or a system error. If you know the data is not deleted but cannot access it, please look at our support page.

What's the best way to fix this? Feel like I'm missing something simple.