Insufficient instance check: instanceof EntityForm

Problem/Motivation

In EntityTypeFormHelper::processVariables there's following part:

if (!$formObject instanceof EntityForm) {
   return;
}

This leads to problems with e.g. the form_decorator (https://www.drupal.org/project/form_decorator) and a node type's form, where the 'Behavior settings' vertical tab suddenly vanishes. This is because, when decorating a form via form_decorator, the form class changes and (in my case) is not an instance of EntityForm anymore.

Steps to reproduce

Install form_decorator and decorate e.g. a node_type form of an entity, where entity_type_behaviors live -- the behaviors tab will vanish.

Proposed resolution

As, anyways, it's OOP's best practice to check against an Interface (if exists), just do
instanceof EntityFormInterface
instead of
instanceof EntityForm