ElementAPI: Error's out if entry doesn't have assets
Why don't you just check if there is at least one photo? Something along the lines of

'transformer' =&gt; function(EntryModel $entry) {
    return [
        'ID' =&gt; $entry-&gt;id,
        'Year' =&gt; $entry-&gt;year,
        'Manufacturer' =&gt; $entry-&gt;manufacturer,
        'Model' =&gt; $entry-&gt;model,
        'Category' =&gt; (string)$entry-&gt;category[0],
        'Pics' =&gt; !empty($entry-&gt;listingPhotos) ? $entry-&gt;listingPhotos[0]-&gt;url : null,
    ];
}