Error when trying to place a new block: RuntimeException: The subform and parent form must contain the #parents property, which must be an array.

I get the following error message after enabling this module and trying to place a new block of category 'Content embed': "RuntimeException: The subform and parent form must contain the #parents property, which must be an array. Try calling this method from a #process callback instead. in Drupal\Core\Form\SubformState->getParents() (line 76 of /my/server/app/path/public_html/core/lib/Drupal/Core/Form/SubformState.php).