Vuejs treeselect plugin - delay loading does not work via vuex action

Using Vue TreeSelect Plugin to load a nested list of nodes from firebase backend. It’s doc page says,

It’s also possible to have root level options to be delayed loaded. If no options have been initially registered (options: null), vue-treeselect will attempt to load root options by calling loadOptions({ action, callback, instanceId }).

loadOptions (in my App.vue) dispatch vuex action_FolderNodesList, fetches (from firebase) formats (as required by vue-treeselect), and mutates the state folder_NodesList, then tries to update options this.options = this.get_FolderNodesList but this does not seems to work.

Here is the loadOptions method (in app.vue)

loadOptions() {
      let getFolderListPromise = this.$store.dispatch("action_FolderNodesList");
      getFolderListPromise.then(_ => {
        this.options = this.get_FolderNodesList;
      });
    }

Vue errors out with Invalid prop: type check failed for prop "options". Expected Array, got String with value ""

I am not sure what am I doing wrong, why that does not work. A working Codesandbox demo

##Source
###App.vue

<template>
    <div class="section">
        <div class="columns">
            <div class="column is-7">
                <div class="field">
                    <Treeselect 
                      :multiple="true" 
                      :options="options" 
                      :load-options="loadOptions" 
                      :auto-load-root-options="false" 
                       placeholder="Select your favourite(s)..." 
                       v-model="value" />
                    <pre>{{ get_FolderNodesList }}</pre>
                </div>
            </div>
        </div>
    </div>
</template>
<script>
import { mapGetters } from "vuex";

import Treeselect from "@riophae/vue-treeselect";
import "@riophae/vue-treeselect/dist/vue-treeselect.css";

export default {
    data() {
        return {
            value: null,
            options: null,
            called: false
        };
    },
    components: {
        Treeselect
    },
    computed: mapGetters(["get_FolderNodesList"]),

    methods: {
        loadOptions() {
            let getFolderListPromise = this.$store.dispatch("action_FolderNodesList");
            getFolderListPromise.then(_ => {
                this.options = this.get_FolderNodesList;
            });
        }
    }
};
</script>

###Store.js

import Vue from "vue";
import Vuex from "vuex";

Vue.use(Vuex);

export const store = new Vuex.Store({
  state: {
    folder_NodesList: ""
  },

  getters: {
    get_FolderNodesList(state) {
      return state.folder_NodesList;
    }
  },
  mutations: {
    mutate_FolderNodesList(state, payload) {
      state.folder_NodesList = payload;
    }
  },
  actions: {
    action_FolderNodesList({ commit }) {
      fmRef.once("value", snap => {
        var testObj = snap.val();

        var result = Object.keys(testObj).reduce((acc, cur) => {
          acc.push({
            id: cur,
            label: cur,
            children: recurseList(testObj[cur])
          });
          return acc;
        }, []);
        commit("mutate_FolderNodesList", result);
      });
    }
  }
});

Any help is appreciated.

Thanks