Fatal error in widget form due to false entity_type due to using 'field_state'

Problem/Motivation

paragraphs_field_widget_form_build() is executed for every paragraphs field. For a nested paragraph, it's executed for different entity types. First on a node, then on a paragraphs_item. field_form_get_state() seems to work on the entire form, not the 'current part' (the $element). This means a sub paragraph element can change the field state to paragraphs_item for the next field item in the node.

Horrible explanation, sorry. Specifically, my problem:

1. paragraph 1 (delta 0 in field X) runs on 'node'
2. paragraph 2 (delta 1 in field X) runs on 'node'
3. paragraph 2 loads a paragraph field Y with 1 item
4. paragraph 3 (delta 0 on field Y) runs on 'paragraphs_item'
5. paragraph 4 (delta 2 in field X) runs on 'paragraphs_item', because the last field state was a 'paragraphs_item', but the current field is a 'node' field, so this is very wrong

It dies on

$paragraph_item->setHostEntity($field_state['instance']['entity_type'], $element['#entity'], $langcode, FALSE);

because $field_state['instance']['entity_type'] is wrong.

Proposed resolution

Don't use $field_state to find the current entity type. You don't use it to find the current entity either. The current entity in $element is correct, so use $element to find the current entity type too:

$paragraph_item->setHostEntity($element['#entity_type'], $element['#entity'], $langcode, FALSE);

Remaining tasks

Change 1 line. Patch later.

User interface changes

None.

API changes

None.

Data model changes

None.