TypeError: Drupal\rest_export_nested\Plugin\views\display\RestExportNested::flatten(): Argument #1 ($data) must be of type array, null given

I am attaching a patch for some interim fixes.

The suggested patch in comment #2 does fix the error but does not make the module usable.
Please check the merge request - added ability to customize rows property, for example, module views_rest_serializer_extra uses "results" instead of "rows".

You can use it as a patch like this:
https://git.drupalcode.org/project/rest_export_nested/-/merge_requests/4...

#5 see the response to your issue #3460838-3: Allow rows name property to be customizable. → (Views REST Serializer Extra ):

This module does allow changing the key names FYI:

@Sivaji_Ganesh_Jojodae, please fill in the section "Steps to reproduce" in the issue summary. It needs more information than just "Some of our views".

Not sure why this is postponed and why it needs more info. Views display plugins can return anything as a row property, it can be any contrib or custom module, and there is @todo in code to make it customizable - Merge Request adds this feature.