Undefined property: stdClass::$fid

I run exactly the same issue, I temporary resolved it by replacing:

function media_gallery_filter_media_access($media_files, $operation) {
  $media_accessable = array();
  foreach ($media_files as $key => $media) {
    if (file_entity_access($operation, (object)$media)) {
      $media_accessable[] = $media;
    }
  }
  return $media_accessable;
}

in media_gallery.module, by

function media_gallery_filter_media_access($media_files, $operation) {
  $media_accessable = array();
  foreach ($media_files as $key => $media) {
    if (isset($media->fid) && file_entity_access($operation, (object)$media)) {
      $media_accessable[] = $media;
    }
  }
  return $media_accessable;
}

Of course, the file is not displayed anymore...

Created patch for the above.