[core] fix appear event on root node

weex_core/Source/core/data_render/vnode/vcomponent.h

@@ -52,7 +52,10 @@ class VComponent : public VNode {
   void MoveTo(VComponent* new_component);
 
   inline const std::string render_object_ref() const override {
-    return "_component_" + std::to_string(id_);
+    if (root_vnode_.get()) {
+      return root_vnode_->render_object_ref();
+    }
+    return "";
   }
 
   inline int id() { return id_; }

weex_core/Source/core/data_render/vnode/vnode_render_context.h

@@ -74,6 +74,9 @@ class VNodeRenderContext {
   }
 
   inline void AddVNode(const std::string& ref, VNode* node) {
+    if (node->IsVirtualComponent()) {
+      return;
+    }
     auto it = vnode_trees_.find(ref);
     if (it != vnode_trees_.end()) {
       vnode_trees_[ref] = node;