Interrapt G22

Description:
When user selects all Attendees by clicking on the square under export button, or user marks as selected only several attendees, and then tries do export only selected attendees on Excel or CSV file, error message '500 SERVER ERROR' is displayed

Steps to reproduce:
1. Navigate to the url 'https://qainterrapt.brainster.xyz/attendees'
2. Select all or just several attendees to be exported
3. Click on the 'Export' button
4. Click 'Selected' under Excel or CSV section

Expected result:
Excel/CSV file that contains the info for ID, Name, Surname, Webinar and Created on for selected attendees should be downloaded

Actual result:
Error message '500 SERVER ERROR' is displayed and the file is not exported