Describe the bug
When you try to import an Assemblie, if the boxes Import Attachments and/or Import Categories are checked you have an error wether the Assemblie you are trying to import have or doesn't have Categories and/or Attachments

To Reproduce
Steps to reproduce the behavior:

1. Go to an Assemblie and export it
2. Go to Assemblie panel and click on Import
3. Check Import Attachments and/or Import Categories
4. Click on 'Import'
5. See error

Expected behavior
We should be able to import Assemblies that have or haven't Categories and/or Attachments if we checked the boxes Import Attachments and/or Import Categories

Screenshots
Importing an Assemblie

Error on try.decidim.org

Error on dev in English

Error on dev in French

Stacktrace
If applicable, add the error stacktrace to help explain your problem.

Extra data (please complete the following information):

• Device: Desktop
• Device OS: Windows 10
• Browser: Chrome
• Decidim Version: 0.27.1 and try.decidim.org

Additional context
Add any other context about the problem here. For instance, add Metadecidim link.