Hi!!

I have now an issue with the what ID element on the event form.
it works for Events related to Opportunitunities but it doesn't work for the Events related to an Account
,
I have added the argument in my trigger but there must be a mistake ..
"Error: Invalid Data.
Review all error messages below to correct your data.
Apex trigger Trigger_Event_Send_Email caused an unexpected exception, contact your administrator: Trigger_Event_Send_Email: execution of BeforeUpdate caused by: System.NullPointerException: Attempt to de-reference a null object: Trigger.Trigger_Event_Send_Email: line 41, column 1

Trigger is :

trigger Trigger_Event_Send_Email on Event (before update) {
    Set<Id> ownerIds = new Set<Id>();
    Set<Id> initiatorIds = new Set<Id>();
    Set<Id> OpptyIds = New Set<Id>();
    Set<Id> AccIds = New Set<Id>();

   
    for(Event evt: Trigger.New)
    //Assigned TO
        ownerIds.add(evt.OwnerId);
    Map<Id, User> userMap = new Map<Id,User>([select Name, Email from User where Id in :ownerIds]);
  
    For(Event evt: Trigger.New)
    //Created By
        initiatorIds.add(evt.CreatedById);
    Map<Id, User> userMap2 = new Map<Id,User>([select Name, Email from User where Id in :initiatorIds]);
  
    For(Event evt : Trigger.New)
  //WhatId = Action
        OpptyIds.add(evt.WhatId);
    Map<id, Opportunity> OpptyMap = new Map<Id,Opportunity>([select Name from Opportunity where Id in :OpptyIds]);
    
    For(Event evt : Trigger.New)
  //WhatId = Account
        AccIds.add(evt.WhatId);
    Map<id, Account> AccMap = new Map<Id,Account>([select Name from Account where Id in :AccIds]);
    
    

    for(Event evt : Trigger.New)  {
        User User = userMap2.get(evt.CreatedById);
        User Assignee = userMap.get(evt.ownerId); 
        Opportunity RelatedTo = OpptyMap.get(evt.WhatId);       
        Account RelatedTo2 = AccMap.get(evt.WhatId);       
        Messaging.SingleEmailMessage mail = new Messaging.SingleEmailMessage();
        String[] toAddresses = new String[] {Assignee.Email};
        mail.setToAddresses(toAddresses);    // Set the TO addresses
        mail.setSubject('A visit Report owned by you is due soon');    // Set the subject

      String Whatid ; 
       if (RelatedTo.Name == '')
       whatid = RelatedTo2.Name;
       
       if (RelatedTo2.Name == '')
       whatid = RelatedTo.Name;

        String template = 'Hello {0}, \nThe following Visit Report is due soon:\n';
        
        String duedate = '';       
        template+= '\n';
        template+= 'Due Date - {1}\n';
        template+= 'Type - {2}\n';     
        template+= 'Subject - {3}\n';
        template+= 'Assign To - {4}\n';
        template+= 'Related To - {5}\n';   
        template+= '\n';
        template+='Click on the following link to access the Visit Report:\n';
        template+= '{6}\n';
        
        //String duedate = '';
    If(System.now() == evt.Send_Email_Alert__c )   
         evt.SendEmailAlert__c = True;
        if (evt.EndDateTime ==null)
            duedate = '';
        Else
           duedate = evt.EndDateTime.format();
                  
               List<String> args = new List<String>();

        args.add(Assignee.Name);
        args.add(duedate);
        args.add(evt.type);
        args.add(evt.subject);
        args.add(Assignee.Name);
        args.add(whatid);
        args.add('https://'+System.URL.getSalesforceBaseURL().getHost()+'/'+evt.Id);
        
        // Here's the String.format() call.
        String formattedHtml = String.format(template, args);
       
        mail.setPlainTextBody(formattedHtml);
        Messaging.SendEmail(new Messaging.SingleEmailMessage[] {mail});
     
            
            }
    }

many thanks!!