Check for the existence of array key before using it in Protocol\ActivityPub\Transmitter

- Address https://github.com/friendica/friendica/issues/9252#issuecomment-739534960

src/Module/Objects.php

@@ -103,6 +103,10 @@ class Objects extends BaseModule
 
 		if (empty($parameters['activity']) && ($item['gravity'] != GRAVITY_ACTIVITY)) {
 			$activity = ActivityPub\Transmitter::createActivityFromItem($item['id'], true);
+			if (empty($activity['type'])) {
+				throw new HTTPException\NotFoundException();
+			}
+
 			$activity['type'] = $activity['type'] == 'Update' ? 'Create' : $activity['type'];
 
 			// Only display "Create" activity objects here, no reshares or anything else

src/Protocol/ActivityPub/Transmitter.php

@@ -1003,10 +1003,10 @@ class Transmitter
 	 * @param integer $item_id
 	 * @param boolean $object_mode Is the activity item is used inside another object?
 	 *
-	 * @return array of activity
+	 * @return false|array
 	 * @throws \Exception
 	 */
-	public static function createActivityFromItem($item_id, $object_mode = false)
+	public static function createActivityFromItem(int $item_id, bool $object_mode = false)
 	{
 		Logger::info('Fetching activity', ['item' => $item_id]);
 		$item = Item::selectFirst([], ['id' => $item_id, 'parent-network' => Protocol::NATIVE_SUPPORT]);