Fix editable point item

lib/timeline/component/item/Item.js

@@ -327,7 +327,13 @@ Item.prototype._repaintDragCenter = function () {
     dragCenter.className = 'vis-drag-center';
     dragCenter.dragCenterItem = this;
 
-    this.dom.box.appendChild(dragCenter);
+    if (this.dom.box) {
+      this.dom.box.appendChild(dragCenter);
+    } 
+    else if (this.dom.point) {
+      this.dom.point.appendChild(dragCenter);
+    }
+    
     this.dom.dragCenter = dragCenter;
   }
   else if (!this.selected && this.dom.dragCenter) {