Exception when clicking the (i) button on an item

[oransterf]

Context
Using the official docker image with ES backend.

Describe the bug

Steps To Reproduce

1. Go to a sist2 frontend for a ES index
2. Click on the (i) icon of any item
3. The item modal pops up but the blue progress bar is stuck

The exception happens at this line:

Example object that is missing object._source.index