"ExecutionException: List index out of bounds: 0" while sending email from Apex using email template with Custom Object merged fields

Question

Here is the code-

public static void sendEmailToUser(String email, String emailTemplateNameText, String selectedRecordId){
    
    String emailTemplateName = '%' + emailTemplateNameText + '%';
    String emailForQuery = '%' + email + '%';
    
    system.debug('selectedRecordId: '+selectedRecordId);
    EmailTemplate emailTemplate = [SELECT Name, DeveloperName, Id, Body 
                                   FROM EmailTemplate 
                                   WHERE Name like :emailTemplateName
                                   AND IsActive = true
                                   LIMIT 1];
    Contact dummyContact = [SELECT Id, Name, Email 
                            FROM Contact
                            WHERE Name <> null
                            AND Email <> null 
                            limit 1];
    User userRec = [SELECT Id, name, Username, email
                    FROM User
                    where Email like :emailForQuery
                    limit 1];
    OrgWideEmailAddress owa = [SELECT id, DisplayName, Address 
                               FROM OrgWideEmailAddress 
                               limit 1];
    
    System.debug('EmailTemplate: '+emailTemplate.Id);
    System.debug('email: '+email);
    System.debug('dummyContact.Id: '+dummyContact.Id);
    System.debug('selectedRecordId: '+selectedRecordId);
    
    String templateId   = emailTemplate.Id;
    String userId       = userRec.Id;
    String whatId       = selectedRecordId;
    
    Messaging.SingleEmailMessage email = new Messaging.SingleEmailMessage();
    email.setTemplateId(emailTemplate.id);
    email.setTargetObjectId(dummyContact.Id);
    email.setWhatId(selectedRecordId);
    email.saveAsActivity = false;
    List<string> toAddress = new List<string>();
    toAddress.add(email);
    email.setToAddresses(toAddress);
    
    if(!Test.isRunningTest()) { 
        try {
            Messaging.SendEmailResult[] sendEmailResult = Messaging.sendEmail(new Messaging.SingleEmailMessage[] {email});
            
            for(Messaging.SendEmailResult r : sendEmailResult){
                if (r.success) {
                    System.debug('Successfully sent email to '+toAddress[0]);
                }else{
                    System.debug('The email failed to send: ' + r.errors[0].message);
                }
            }
        } catch (Exception e) {
            System.debug(e.getMessage());
        }
    }
}

The debug logs for this method reflect that the System debugs all have the correct values. The merge fields are also populating correctly, and the debug log at the success also executes. However the email isn't sent and I get the above exception. I'm using a custom email template for this example, if that might be an issue.