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add_action( 'save_post', 'servico_imagem_destacada' );
function servico_imagem_destacada($post_id) {

  if ( 'evento' == get_post_type() ) {
    if (get_post_thumbnail_id($post_id) == null){
      $id_espetaculo = get_field('espetaculo_id', $post_id);
      $id_thumbnail_especaculo = get_post_thumbnail_id($id_espetaculo);
      update_post_meta( $post_id, '_thumbnail_id', $id_thumbnail_especaculo );
    }
  }
	
}