Undefined array key "field_name" in Drupal\selective_better_exposed_filters\Plugin\better_exposed_filters\filter\SelectiveFilterBase::exposedFormAlter()

Attached patch.

Changes from the #2 patch fixed the warning message

Sorry, but the proposed patch will create another notice at the next lines when the variable is just undefined.
I've added a bit another fix, check please. I hope it will solve your problem.

Automatically closed - issue fixed for 2 weeks with no activity.