Error handling in custom wp_query script

I found this script somewhere that connects to another WP-instance and gets 1 post with a certain tag.

?php global $switched;
switch_to_blog(1); ?>
<?php
$query = new WP_Query (array( 'post_type' => array( 'events'), 'tag' => 'gezondheidszorg', 'showposts' => '1', 'order' => 'DESC'));
$months = array("", "januari", "februari", "maart", "april", "mei", "juni", "juli", "augustus", "september", "oktober", "november", "december");
function sortFunction( $a, $b ) {
	return $a["date"] - $b["date"];
}
while ( $query->have_posts() ) : $query->the_post();
$date = get_post_meta(get_the_ID(), 'date');
$id = get_the_ID();
$tosortposts []= array(
'date' => strtotime($date[0]),
'link' => get_permalink($id), 
'title' => get_the_title($id)
);
endwhile;
usort($tosortposts, "sortFunction");
foreach($tosortposts as $p){
	echo '<strong>Onze volgende workshop:</strong> <a target="_blank" href="'; echo $p['link']; echo '">';
	echo date('j ', $p['date']).$months[date("n", $p['date'])].' - ';
	echo ''; echo $p['title']; echo '</a><p></p><p>';	
	echo '</p>';
}
?>	
</div>
<?php restore_current_blog(); ?>

This script is working just fine as long there is an item with that tag.
But when there is no item with that tag i get these errors:

Warning: usort() expects parameter 1 to be array, null given in page-home.php on line 68
Warning: Invalid argument supplied for foreach() in page-home.php on line 69

I guess i have to make some sort of error handling, but I am not sure where to start.
Anyone can lend help on this one?