bugfix: when $attrs is updated before mounted, throw: Uncaught (in promise) TypeError: Cannot read property 'option' of undefined. Because the variable _sortable has not been initialized.

Author: muwoo

src/vuedraggable.js

@@ -165,6 +165,7 @@ const draggableComponent = defineComponent({
     $attrs: {
       handler(newOptionValue) {
         const { _sortable } = this;
+        if (!_sortable) return;
         getValidSortableEntries(newOptionValue).forEach(([key, value]) => {
           _sortable.option(key, value);
         });