Use Vue.set to ensure that the updated pack property is reactive

If it was null, it wouldn't be reactive when editing

src/store/modules/emoji_packs.js

@@ -2,6 +2,8 @@ import { listPacks, downloadFrom, reloadEmoji, deletePack, savePackMetadata } fr
 
 import { Message } from 'element-ui'
 
+import Vue from 'vue'
+
 const packs = {
   state: {
     localPacks: {},
@@ -16,7 +18,7 @@ const packs = {
     },
 
     UPDATE_LOCAL_PACK_VAL: (state, { name, key, value }) => {
-      state.localPacks[name]['pack'][key] = value
+      Vue.set(state.localPacks[name]['pack'], key, value)
     },
 
     UPDATE_LOCAL_PACK: (state, { name, pack }) => {