Throw an exception where category doesn't exist

Author: ldusan84

app/code/Magento/CatalogGraphQl/Model/Resolver/CategoryTree.php

@@ -11,6 +11,7 @@
 use Magento\Framework\GraphQl\Schema\Type\ResolveInfo;
 use Magento\Framework\GraphQl\Config\Element\Field;
 use Magento\Framework\GraphQl\Exception\GraphQlInputException;
+use Magento\Framework\GraphQl\Exception\GraphQlNoSuchEntityException;
 use Magento\Framework\GraphQl\Query\ResolverInterface;
 
 /**
@@ -72,11 +73,12 @@ public function resolve(Field $field, $context, ResolveInfo $info, array $value
 
         $rootCategoryId = $this->getCategoryId($args);
         $categoriesTree = $this->categoryTree->getTree($info, $rootCategoryId);
-        if (!empty($categoriesTree) && ($categoriesTree->count() > 0)) {
-            $result = $this->extractDataFromCategoryTree->execute($categoriesTree);
-            return current($result);
-        } else {
-            return null;
+
+        if (empty($categoriesTree) || ($categoriesTree->count() == 0)) {
+            throw new GraphQlNoSuchEntityException(__('Category doesn\'t exist'));
         }
+
+        $result = $this->extractDataFromCategoryTree->execute($categoriesTree);
+        return current($result);
     }
 }

dev/tests/api-functional/testsuite/Magento/GraphQl/Catalog/CategoryTest.php

@@ -123,7 +123,7 @@ public function testNonExistentCategoryWithProductCount()
 QUERY;
 
         $response = $this->graphQlQuery($query);
-        $expectedResponse = ['category' => null];
+        $expectedResponse = ['errors' => ['message' => 'Category doesn\'t exist']];
         $this->assertEquals($expectedResponse, $response);
     }