Undefined variable in version 2.1.4

On mobile devices the gallery still produces an “Notice: Undefined variable: output …” error.

Here is the fix:

--- polaroid_gallery.php.sav 2014-12-08 03:12:18.000000000 +0000
+++ polaroid_gallery.php        2014-12-13 00:52:11.000000000 +0000
@@ -379,7 +379,7 @@
 }

 function makeGalleryForPhones($attachments, $thumbnail_caption, $id, $image_size){
-       $output .= "<div class='polaroid-gallery galleryid-{$id}'>";
+       $output = "<div class='polaroid-gallery galleryid-{$id}'>";

        $caption_class = '';
        if($thumbnail_caption == 'show') {

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