Argument #1 ($form_values) must be of type array, null given

I fixed this bug by simply checking, if an array with the form field name as key name exists.

Additionally I addressed also following error:

Deprecated function: Creation of dynamic property Drupal\group_by_field_widget\Plugin\Field\FieldWidget\GroupByFieldReferenceWidget::$groupableFields is deprecated in Drupal\group_by_field_widget\Plugin\Field\FieldWidget\GroupByFieldReferenceWidget->__construct() (line 79 of modules/contrib/group_by_field_widget/src/Plugin/Field/FieldWidget/GroupByFieldReferenceWidget.php).

I was too lazy too create another issue for such a simple thing.

The issue is reproducible with the module version 1.2.3 and the patch fixes the issue, and the group by field reference widget form display now can be used for entity reference field, which, was earlier got stuck in ajax loop.

That you StefanieB, your patch has been applied to the latest release.