[Solved] UNKNOWN_EXCEPTION, List has no rows

Miguel Asks: UNKNOWN_EXCEPTION, List has no rows
I have the following error when trying to send an email.

System.EmailException: SendEmail failed. First exception on row 0; first >error: UNKNOWN_EXCEPTION, List has no rows for assignment to SObject: []

    List<User> users = [SELECT Name, Email, Id FROM User WHERE Recipient__c = TRUE and isActive = TRUE];
    EmailTemplate et = [SELECT id FROM EmailTemplate WHERE DeveloperName='Advisor'];

    for(User u : users) {
        Messaging.SingleEmailMessage mail = new Messaging.SingleEmailMessage();


        Messaging.SendEmailResult[] r = Messaging.sendEmail(new Messaging.SingleEmailMessage[] {mail});

The list users contains at least one record and the Emailtemplate is not null.

This is the result of the mail system debug:

(Messaging.SingleEmailMessage[ getBccAddresses=null; getCcAddresses=null; getCharset=null; getEntityAttachments=null; getFileAttachments=null; getHtmlBody=null; getInReplyTo=null; getOptOutPolicy=SEND; getOrgWideEmailAddressId=null; getPlainTextBody=null; getReferences=null; getTargetObjectId=005D0XXXXXXXXXuIAE; getTemplateId=00XD0XXXXXXXXcbMAA; getToAddresses=null; getWhatId=null; isTreatBodiesAsTemplate=false; isTreatTargetObjectAsRecipient=true; isUserMail=true; ])

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