我有以下两张table：


我需要添加ut作为id\u ut的外键，但是“referenced column”中的条目消失了。

在我看来，键值的所有约束都得到了满足（数据类型、大小、唯一性）。这里怎么了？。谢谢您。
下面是一个最简单的工作示例：

-- MySQL dump 10.13  Distrib 5.7.17, for Win32 (AMD64)
--
-- Host: 127.0.0.1    Database: newschema
-- ------------------------------------------------------
-- Server version	5.7.17
/*!40101 SET @OLD_CHARACTER_SET_CLIENT=@@CHARACTER_SET_CLIENT */;
/*!40101 SET @OLD_CHARACTER_SET_RESULTS=@@CHARACTER_SET_RESULTS */;
/*!40101 SET @OLD_COLLATION_CONNECTION=@@COLLATION_CONNECTION */;
/*!40101 SET NAMES utf8 */;
/*!40103 SET @OLD_TIME_ZONE=@@TIME_ZONE */;
/*!40103 SET TIME_ZONE='+00:00' */;
/*!40014 SET @OLD_UNIQUE_CHECKS=@@UNIQUE_CHECKS, UNIQUE_CHECKS=0 */;
/*!40014 SET @OLD_FOREIGN_KEY_CHECKS=@@FOREIGN_KEY_CHECKS, FOREIGN_KEY_CHECKS=0 */;
/*!40101 SET @OLD_SQL_MODE=@@SQL_MODE, SQL_MODE='NO_AUTO_VALUE_ON_ZERO' */;
/*!40111 SET @OLD_SQL_NOTES=@@SQL_NOTES, SQL_NOTES=0 */;
--
-- Table structure for table `catalogo_reperti`
--
DROP TABLE IF EXISTS `catalogo_reperti`;
/*!40101 SET @saved_cs_client     = @@character_set_client */;
/*!40101 SET character_set_client = utf8 */;
CREATE TABLE `catalogo_reperti` (
  `UT` varchar(45) COLLATE utf16_unicode_ci NOT NULL,
  `Id_Reperto` varchar(45) COLLATE utf16_unicode_ci NOT NULL,
  `Documentazione` text COLLATE utf16_unicode_ci,
  PRIMARY KEY (`Id_Reperto`,`UT`),
  UNIQUE KEY `Id_Reperto_UNIQUE` (`Id_Reperto`)
) ENGINE=InnoDB DEFAULT CHARSET=utf16 COLLATE=utf16_unicode_ci;
/*!40101 SET character_set_client = @saved_cs_client */;
--
-- Table structure for table `catalogo_unita_topografiche`
--
DROP TABLE IF EXISTS `catalogo_unita_topografiche`;
/*!40101 SET @saved_cs_client     = @@character_set_client */;
/*!40101 SET character_set_client = utf8 */;
CREATE TABLE `catalogo_unita_topografiche` (
  `Id_UT` varchar(45) COLLATE utf16_unicode_ci NOT NULL,
  PRIMARY KEY (`Id_UT`),
  UNIQUE KEY `Id_UT_UNIQUE` (`Id_UT`)
) ENGINE=InnoDB DEFAULT CHARSET=utf16 COLLATE=utf16_unicode_ci;
/*!40101 SET character_set_client = @saved_cs_client */;
/*!40103 SET TIME_ZONE=@OLD_TIME_ZONE */;
/*!40101 SET SQL_MODE=@OLD_SQL_MODE */;
/*!40014 SET FOREIGN_KEY_CHECKS=@OLD_FOREIGN_KEY_CHECKS */;
/*!40014 SET UNIQUE_CHECKS=@OLD_UNIQUE_CHECKS */;
/*!40101 SET CHARACTER_SET_CLIENT=@OLD_CHARACTER_SET_CLIENT */;
/*!40101 SET CHARACTER_SET_RESULTS=@OLD_CHARACTER_SET_RESULTS */;
/*!40101 SET COLLATION_CONNECTION=@OLD_COLLATION_CONNECTION */;
/*!40111 SET SQL_NOTES=@OLD_SQL_NOTES */;