After our upgrade to Umbraco 10 and Vendr 3.0.3 we've been getting the following error when finalizing an order from within the backend code.

System.AggregateException: AsyncHelper.RunSync method threw an exception. (Value cannot be an empty array. (Parameter 'to'))
 ---> System.ArgumentException: Value cannot be an empty array. (Parameter 'to')
   at Umbraco.Cms.Core.Models.Email.EmailMessage.ArgumentIsNotNullOrEmpty(String[] arg, String argName)
   at Umbraco.Cms.Core.Models.Email.EmailMessage..ctor(String from, String[] to, String[] cc, String[] bcc, String[] replyTo, String subject, String body, Boolean isBodyHtml, IEnumerable`1 attachments)
   at Vendr.Extensions.MailMessageExtensions.ToEmailMessage(MailMessage message)
   at Vendr.Umbraco.Mail.UmbracoEmailSender.SendAsync(MailMessage message)
   at Vendr.Core.Pipelines.Email.Tasks.SendEmailTask.<>c__DisplayClass3_0.<Execute>b__0()
   at Vendr.Common.Helpers.AsyncHelper.<>c__DisplayClass1_0.<<RunSync>b__0>d.MoveNext()
   --- End of inner exception stack trace ---
   at Vendr.Infrastructure.Resiliency.PollyExecutionStrategyBase.Execute[TResult](Func`1 operation, Func`1 verifySucceeded)
   at Vendr.Core.Services.EmailTemplateService.SendEmail[TModel](EmailTemplateReadOnly emailTemplate, TModel model, String toEmailAddress, String languageIsoCode)
   at Vendr.Core.Services.EmailTemplateService.SendEmail(EmailTemplateReadOnly emailTemplate, OrderReadOnly order)
   at Vendr.Core.Events.Notification.Handlers.Order.SendFinalizedOrderEmail.Handle(OrderFinalizedNotification evt)

We use the following code for finalizing an order from the backend:

OrderReadOnly? order = null;

var store = this.vendrApi.GetStore(Constants.StoreAlias);

var currentOrder = this.vendrApi
    .GetOrCreateCurrentOrder(store.Id);

this.vendrApi.Uow.Execute(uow =>
{
    var writableOrder = currentOrder.AsWritable(uow);

    var properties = new Dictionary<string, string>
    {
        { Vendr.Core.Constants.Properties.Customer.EmailPropertyAlias, emailAddress },
        { Vendr.Core.Constants.Properties.Customer.FirstNamePropertyAlias, firstName },
        { Vendr.Core.Constants.Properties.Customer.LastNamePropertyAlias, lastName },
    };

    writableOrder.SetProperties(properties);

    var paymentMethod = this.vendrApi.GetPaymentMethod(store.Id, paymentMethodAlias);
    writableOrder.SetPaymentMethod(paymentMethod);

    order = writableOrder.InitializeTransaction(this.orderNumberGenerator);
    writableOrder.Finalize(0, currentOrder.TransactionInfo.TransactionId, PaymentStatus.Captured);

    this.vendrApi.SaveOrder(writableOrder);

    uow.Complete();
}

What do I need to change to not get that error anymore?

This bit of code is used in a userflow where we've been fine with not sending order confirmation emails in the past. It did not send emails, but it also did not complain in the logs about it.