I pushed a trigger live a couple of weeks ago and it seemed to be working fine. But in the last two days, I've received 11 emails notifying me of an error. I've tested and can't duplicate the error and I'm not sure what's causing it. I think it must be an automated process that's trying to create an event because none of my users have complained about the error. How can I determine what is causing the error and how can I fix it?

Here's the error: 

Trigger.EventIndustry: line 28, column 38

Apex script unhandled trigger exception by user/organization: 00560000000m32k/00D600000006nRH

EventIndustry: execution of BeforeInsert

caused by: System.NullPointerException: Attempt to de-reference a null object

Here's the code:

trigger EventIndustry on Event (before insert, before update) {
	if(trigger.isInsert){
        //create a set of all unique WhatIDs and WhoIDs
        set<id> WhoIDs = new Set<id>();
        Set<id> WhatIds= new Set<id>();
        Set<id> AccountIds= new Set<id>();
        
        for (Event t : trigger.new){
        	if (String.valueOf(t.whoId)!= null){
               	WhoIDs.add(t.WhoID); 
        	}else if (String.valueOf(t.WhatID)!= Null){
        		WhatIds.add(t.WhatID);
         	}else if (String.valueOf(t.AccountID)!= Null){
        		AccountIds.add(t.AccountID);
        	}
        }   
        //create a map for a hash table for the industry
       	Map<id, Lead> who = new Map<id, Lead>([SELECT Industry FROM Lead WHERE ID in :WhoIDs]);     
	   	Map<Id, Account> acct = new Map<Id, Account>([Select Industry From Account Where ID in :AccountIds OR ID in: WhatIDs]);
	   	Map<id, Contact> con = new Map<id, Contact>([SELECT Account.Industry from Contact WHERE ID in :WhoIDs]);
	   	Map<id, Opportunity> opp = new Map<id, Opportunity>([Select Account.Industry From Opportunity Where Id in :WhatIDs]);	
		
		// iterate over the list of records being processed in the trigger and set the industry
		for (Event a : Trigger.new){
			if (!who.isEmpty()){
		    	a.Industry__c = who.get(a.WhoId).Industry;
			} else if (!con.isEmpty()){
				a.Industry__c = con.get(a.WhoId).account.Industry;
			} else if (!opp.isEmpty()){
				a.Industry__c = opp.get(a.WhatId).account.Industry;
			} else if (acct != null && !acct.isEmpty()){
					if(acct.ContainsKey(a.whatID)){
						a.Industry__c = acct.get(a.whatID).Industry;
					}else if(acct.ContainsKey(a.AccountId)){
						a.Industry__c = acct.get(a.AccountId).Industry;
					}
			}
		}
	} else {
		if(trigger.isUpdate){
			 //create a set of all unique ids 
	        set<id> WhoIDs = new Set<id>();
	        Set<id> WhatIds= new Set<id>();
	        Set<id> AccountIds= new Set<id>();
			
			for (Event t : Trigger.new){
				if (Trigger.oldMap.get(t.Id).WhoId != Trigger.newMap.get(t.Id).WhoId){
					WhoIDs.add(t.WhoID); 
				}else if (Trigger.oldMap.get(t.Id).WhatID != Trigger.newMap.get(t.Id).WhatID){
					WhatIds.add(t.WhatID);
				}else if (Trigger.oldMap.get(t.Id).AccountID != Trigger.newMap.get(t.Id).AccountID){
					AccountIDs.add(t.AccountID);
				}
			}
			
			//create a map for a hash table for the industry
       		Map<id, Lead> who = new Map<id, Lead>([SELECT Industry FROM Lead WHERE ID in :WhoIDs]);     
	   		Map<Id, Account> acct = new Map<Id, Account>([Select Industry From Account Where ID in :AccountIds OR ID in: WhatIDs]);
	   		Map<id, Contact> con = new Map<id, Contact>([SELECT Account.Industry from Contact WHERE ID in :WhoIDs]);
	   		Map<id, Opportunity> opp = new Map<id, Opportunity>([Select Account.Industry From Opportunity Where Id in :WhatIDs]);	
			
			for (Event a : Trigger.new){
				if (!who.isEmpty()&& who.ContainsKey(a.WhoId)){
			    	a.Industry__c = who.get(a.WhoId).Industry;
				} else if (!con.isEmpty()){
					a.Industry__c = con.get(a.WhoId).account.Industry;
				} else if (!opp.isEmpty()){
					a.Industry__c = opp.get(a.WhatId).account.Industry;
				} else if (acct != null && !acct.isEmpty()){
					if(acct.ContainsKey(a.whatID)){
						a.Industry__c = acct.get(a.whatID).Industry;
					}else if(acct.ContainsKey(a.AccountId)){
						a.Industry__c = acct.get(a.AccountId).Industry;
					}
				}
			}
		}		
	}
}

Thanks,

Wendy