"Undefined index: override" and "Undefined index: target_id" when saving field configuration

When creating or editing an ER Override field on Drupal 8.9.1 on PHP 7.2.31, several PHP notices appear.

The following message is repeated twice:

Notice: Undefined index: override in entity_reference_override_validate_custom_text() (line 58 of modules/contrib/entity_reference_override/entity_reference_override.module).

The following message is repeated four times:

Notice: Undefined index: target_id in Drupal\Core\Field\Plugin\Field\FieldWidget\EntityReferenceAutocompleteWidget->massageFormValues() (line 144 of core/lib/Drupal/Core/Field/Plugin/Field/FieldWidget/EntityReferenceAutocompleteWidget.php).