Notice: Undefined index: fieldname in ReferenceSelectWidget::formElement()

Will create a patch later, both these seem to work for me in first tests:

$target_id = NestedArray::getValue($user_input, array_merge($element['target_id']['#field_parents'], [$field_name, $delta, 'target_id']));
$target_id = $form_state->getValue(array_merge($element['target_id']['#field_parents'], [$field_name, $delta, 'target_id']))

I saw that has been committed already since then. It does indeed fix the notice, but I don't think it's a proper fix. It will simply not find the value in the user input values, which means it will _not_ update to use the correct webform information then. Also, there's no reason for multiple issets, one would have been enough, it's smart enough to check all the keys.

My fix based on the parents tree should find it in case it is nested as well and not fail if a key isn't defined in case of access.

Here's a patch that uses $form_state->getValue(), I think that should work, we'll see if tests disagree.

Also reverted the isset stuff, not necessary anymore in the first case and never was in the second. we already access $delta earlier, so that has to exist and ->target_id is a field property, that also can't not exist if the delta exists. And the function will not be called with an on-existing delta.