Backporting preview feature of privatemsg to 4.7 but getting blank page saying "Array"

I've upgraded to the latest 4.7 release of the privatemsg module, then I tried backporting the "preview" feature of the 5.x release to mt 4.7.x install

I tried modifying the privatemsg.module as per: http://cvs.drupal.org/viewvc.py/drupal/contributions/modules/privatemsg/...

Here is the code I put in, in case someone can spot something obvious problems with it on a 4.7 installation:

$form = array('preview' => array());

[move forward some lines...]

$form[] = array(
'#type' => 'submit',
'#value' => t('Preview')
);
$form[] = array(

[move forward some lines...]

$form['#after_build'] = array('_privatemsg_new_preview');
return $form;
}

[move forward some lines...]

function _privatemsg_new_preview($form, $form_values) {
  if ($form_values['op'] == t('Preview')) {
    $recipient = user_load(array('name' => $form_values['recipient']));
    if ($recipient->uid) {
      global $user;
      $message->subject = $form_values['subject'];
      $message->message = $form_values['privatemsgbody'];
      $message->format = $form_values['format'];
      message->timestamp = time();
      $message->newmsg = TRUE;
      $message->folder = 1;
      $message->uid = $message->author = $user->uid;
      $message->name = $user->name;
      $message->recipient = $recipient->uid;
      $form['preview'] = array(
        '#prefix' => '<div class="preview">',
        '#value' => theme('privatemsg_view', $message),
        '#suffix' => '</div>');
      drupal_set_title(t('Preview message'));
   }
}

[move forward some lines...]

if ($form_values['op'] != t('Send private message')) {
return false;
}

After adding the code, now when I click on the "Write a new message" or when I reply to a message I get a blank page that says the single word: Array

Any suggestions, please?