Popover: compatible with 2.6 new v-slot syntax (#20424)

packages/popover/src/main.vue

@@ -18,8 +18,10 @@
         <slot>{{ content }}</slot>
       </div>
     </transition>
+    <span class="el-popover__reference-wrapper" ref="wrapper" >
       <slot name="reference"></slot>
     </span>
+  </span>
 </template>
 <script>
 import Popper from 'element-ui/src/utils/vue-popper';
@@ -87,8 +89,8 @@ export default {
     let reference = this.referenceElm = this.reference || this.$refs.reference;
     const popper = this.popper || this.$refs.popper;
 
-    if (!reference && this.$slots.reference && this.$slots.reference[0]) {
-      reference = this.referenceElm = this.$slots.reference[0].elm;
+    if (!reference && this.$refs.wrapper.children) {
+      reference = this.referenceElm = this.$refs.wrapper.children[0];
     }
     // 可访问性
     if (reference) {
@@ -192,8 +194,8 @@ export default {
       let reference = this.reference || this.$refs.reference;
       const popper = this.popper || this.$refs.popper;
 
-      if (!reference && this.$slots.reference && this.$slots.reference[0]) {
-        reference = this.referenceElm = this.$slots.reference[0].elm;
+      if (!reference && this.$refs.wrapper.children) {
+        reference = this.referenceElm = this.$refs.wrapper.children[0];
       }
       if (!this.$el ||
         !reference ||