Issue #2254777 by m1r1k: Block category autocomplete does not work.

core/modules/block/lib/Drupal/block/Controller/CategoryAutocompleteController.php

@@ -59,7 +59,7 @@ public function autocomplete(Request $request) {
     $matches = array();
     foreach ($this->blockManager->getCategories() as $category) {
       if (stripos($category, $typed_category) === 0) {
-        $matches[$category] = String::checkPlain($category);
+        $matches[] = array('value' => $category, 'label' => String::checkPlain($category));
       }
     }
     return new JsonResponse($matches);

core/modules/block/tests/Drupal/block/Tests/CategoryAutocompleteTest.php

@@ -59,8 +59,8 @@ public function setUp() {
    */
   public function testAutocompleteSuggestions($string, $suggestions) {
     $suggestions = array_map(function ($suggestion) {
-      return String::checkPlain($suggestion);
-    }, array_combine($suggestions, $suggestions));
+      return array('value' => $suggestion, 'label' => String::checkPlain($suggestion));
+    }, $suggestions);
     $result = $this->autocompleteController->autocomplete(new Request(array('q' => $string)));
     $this->assertSame($suggestions, json_decode($result->getContent(), TRUE));
   }