Warning: unserialize() expects parameter 1 to be string, array given in _link_load() line 320 when previewing node.

This issue happens when you preview a node that has a link field.
I believe that the attribute is still an array at this stage and is not yet serialised.
One way to fix it is to change the code like this:

 if (isset($item['attributes'])) {
    if(is_string($item['attributes'])){
      return unserialize($item['attributes']);
    } else {
      return $item['attributes'];
    }
  }