Undefined index for #theme and #allow_reference_changes leads to AjaxError when opening drag and drop feature

After some recent updates I noticed that the drag and drop functionality of my paragraphs (stable) widget did break with an AjaxError. The following notices lead me to paragraphs_sets:

Notice: Undefined index: #theme in paragraphs_sets_field_widget_multivalue_form_alter() (Zeile 348 in /app/htdocs/modules/contrib/paragraphs_sets/paragraphs_sets.module)

Notice: Undefined index: #allow_reference_changes in Drupal\paragraphs_sets\ParagraphsSets::buildSelectSetSelection() (Zeile 145 in /app/htdocs/modules/contrib/paragraphs_sets/src/ParagraphsSets.php)

I used isset, which brought drag & drop back. Not sure, if this could affect other parts of the code, though – maybe somebody could check.

core: 8.9.16 / seven admin theme
paragraphs: 8.x-1.12+14-dev
paragraphs_sets: 8.x-2.7+5-dev
paragraphs_features: 8.x-1.11+4-dev
paragraphs_ee: 8.x-1.7+4-dev

Thanks in advance, Anton