Dear Magda,
Sorry for my late response. I have looked into it - indeed, there is a bug, or rather the user experience has not been taken into account correctly. I'll work on it as soon as possible, normally it should be solved in the course of next week.
In the meantime, there is a "quick and dirty" workaround, which involves editing the xml files in a text editor. You can add the "tomo_idx" attribute manually, by using the find-and-replace function of your text editor, with find="<object " and replace="<object tomo_idx="0" ". I understand this is rather fastidious, and hopefully I can provide you with a better solution ASAP.
Furthermore, there will soon be a Napari plugin that will replace the current graphical user interface. We are currently running the last tests before official release.
Best regards,
Emmanuel Moebel