Warning: Undefined array key "#type"

Problem/Motivation

Editing a header slide media entity I get:

Warning: Undefined array key "#type" in Drupal\Core\Form\FormHelper::processStates() (Zeile 211 in /web/core/lib/Drupal/Core/Form/FormHelper.php

looking at the backtrace I see

38: Drupal\Core\Form\FormHelper::processStates() => array (2)

    ⇄file => string (44) "core/lib/Drupal/Core/Form/FormHelper.php:211"

    ⇄⧉args => array (1)

        ⇄⧉0 => array (15)

            ⇄⧉#states => array (1)

                ⇄⧉invisible => array (1)

                    ⇄⧉:input[name="field_overlay_position"] => array (1)

                        ⇄value => string (8) "disabled"

which is build in:
drowl_header_slides.module

So I guess we're indirectly causing this?

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes