Errors on Field update creation

Problem/Motivation

I tried to create a Field Updater, but get errors when I click Save ...

Steps to reproduce

1. Add a new field in Article, called "Text1" and "Text2"
2. Create a new field updater with target "Text2" and click Save
3. Get a lot of errors

Error message
Warning: Attempt to read property "entity_type" on null in Drupal\field_updater_service\Form\FieldUpdaterForm->save() (line 300 of modules/contrib/field_updater_service/src/Form/FieldUpdaterForm.php).
[...]
Warning: Array to string conversion in Drupal\field_updater_service\Form\FieldUpdaterForm->save() (line 310 of modules/contrib/field_updater_service/src/Form/FieldUpdaterForm.php).

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes