I am seeing this error notice on the edit page of a node with Fieldsets. In particular, if the fieldset has a description but no ID attribute, then I get the error.
Here is the message:
Notice: Undefined index: id in template_preprocess_fieldset() (line 217 of core/includes/form.inc).
It looks like the following code is the culprit. So I'm not sure if that's actually a core issue or an issue where the Field Group module needs to require an ID or provide a default one.
$description_id = $element['#attributes']['id'] . '--description';