Get an instance of the paragraph selection handler.
false|\Drupal\paragraphs\Plugin\EntityReferenceSelection\ParagraphSelection The paragraph selection handler.
protected function getParagraphSelectionHandler() {
// Get an instance of the ParagraphsSelection handler.
$options = [
'target_type' => 'paragraph',
'handler' => 'default:paragraph',
];
return $this->pluginManagerEntityReferenceSelection
->getInstance($options);
}