Warning: array_push() expects parameter 1 to be array, null given in /usr/local/apache/common-local/php-1.21wmf1/extensions/Collection/Collection.body.php on line 551

static function addChapter( $name ) {

		$collection = CollectionSession::getCollection();
		array_push( $collection['items'], array(
			'type' => 'chapter',
			'title' => $name,
		) );
		CollectionSession::setCollection( $collection );

}

https://gerrit.wikimedia.org/r/27653