Recoverable fatal error on running the importer with the update existing content settings

Problem/Motivation

The error happened while testing a fresh Drupal 8 install with the patch https://www.drupal.org/files/issues/2020-10-18/feeds_migrate-process-300... by @MegaChriz . I've already reported the issue over there: https://www.drupal.org/project/feeds_migrate/issues/3002362#comment-1387... . But according to @MegaChriz it is probably an unrelated issue to the tamper process therefor he agreed that i better open up a separate one outlining the steps leading to the error.

Steps to reproduce

- I've tried importing a JSON file.
- Mapping a unique_id to ID and name to title.
- For title I've applied a single process plugin to convert case (first character to lowercase)
- On an initial import three nodes were created.
- Now I've added more process plugins to the title (1.characters to trim to remove the spaces within the string 2. a second convert case - just for testing purposes to have more than one process plugin and more than one convert case plugin)
- Changed the importer settings to update existing content
- Clicking save lead to the recoverable fatal error