Errors can always be ignored. 
They are annoying, though.
I think it is a bug.
In Recent.controller.php, find:
// We need to check the topic icons exist... you can never be too sure!
if (!empty($modSettings['messageIconChecks_enable']))
{
// First icon first... as you'd expect.
if (!isset($context['icon_sources'][$row['first_icon']]))
$context['icon_sources'][$row['first_icon']] = file_exists($settings['theme_dir'] . '/images/post/' . $row['first_icon'] . '.png') ? 'images_url' : 'default_images_url';
// Last icon... last... duh.
if (!isset($context['icon_sources'][$row['last_icon']]))
$context['icon_sources'][$row['last_icon']] = file_exists($settings['theme_dir'] . '/images/post/' . $row['last_icon'] . '.png') ? 'images_url' : 'default_images_url';
}
and add immediately after:
else
{
if (!isset($context['icon_sources'][$row['first_icon']]))
$context['icon_sources'][$row['first_icon']] = 'images_url';
if (!isset($context['icon_sources'][$row['last_icon']]))
$context['icon_sources'][$row['last_icon']] = 'images_url';
}
It should do the trick.
If it works, please let us know, so that it can be pushed to 1.0.2.