EntityStorageException when saving existing paragraph revision via Inline Entity Form

Steps to reproduce

1. Enable revisions on a Paragraphs type.
2. Add a Paragraph field to a content type, using the Inline Entity Form (IEF) - Embedded widget.
3. Configure the widget to "allow editing of existing entities".
4. Create a node, add a paragraph, save.
5. Edit the node, modify the paragraph, and save.

Proposed resolution

Force any saved revision to be the default when saving a revisionable entity via IEF.
Suggested fix in `WidgetSubmit::doSubmit()`:

```php
if ($entity instanceof \Drupal\Core\Entity\RevisionableInterface) {
$entity->setNewRevision(TRUE);
$entity->isDefaultRevision(TRUE);
}