I followed a tutorial :

http://tech-jeff.com/blog/magento-2-creating-custom-web-apis

I have an error:

I am trying to get all slides, but I have an error:

My code is:

/**
     * @api
     * @param \Magento\Framework\Api\SearchCriteriaInterface $criteria
     * @return \Magento\Framework\Api\SearchResultsInterface
     */
    public function getList(\Magento\Framework\Api\SearchCriteriaInterface $criteria)
    {
        $searchResults = $this->searchResultsFactory->create();
        $searchResults->setSearchCriteria($criteria);  
        $collection = $this->slideCollectionFactory->create();
        foreach ($criteria->getFilterGroups() as $filterGroup) {
            $fields = [];
            $conditions = [];
            foreach ($filterGroup->getFilters() as $filter) {
                $condition = $filter->getConditionType() ? $filter->getConditionType() : 'eq';
                $fields[] = $filter->getField();
                $conditions[] = [$condition => $filter->getValue()];
            }
            if ($fields) {
                $collection->addFieldToFilter($fields, $conditions);
            }
        }  
        $searchResults->setTotalCount($collection->getSize());
        $sortOrders = $criteria->getSortOrders();
        if ($sortOrders) {
            /** @var SortOrder $sortOrder */
            foreach ($sortOrders as $sortOrder) {
                $collection->addOrder(
                    $sortOrder->getField(),
                    ($sortOrder->getDirection() == SortOrder::SORT_ASC) ? 'ASC' : 'DESC'
                );
            }
        }
        $collection->setCurPage($criteria->getCurrentPage());
        $collection->setPageSize($criteria->getPageSize());
        $objects = [];                                     
        foreach ($collection as $objectModel) {
            $objects[] = $objectModel;
        }
        $searchResults->setItems($objects);
        return $searchResults;        
    }