Vuex state is null in mutation after page refresh instead predefined object

Question

In a simple application, I create a simple set of mutations and actions that are tied to the component creation hook. In browser, after pressing F5, when vue-devtools opened on Vuex tab I get an error at the start of the application, although this should not happen.

Main question: why state is 'null' and how to change it?

store.js

export default new Vuex.Store({
  state: {
    a: undefined,
    b: undefined
  },
  mutations: {
    SET_A (state, a) {
      console.info(a)
      state.a = a // ← store.js?c0d6:14
    },
    SET_B (state, b) {
      state.b = b
    }
  },
  actions: {
    setA ({ commit }, a) {
      console.info(a)
      commit('SET_A', a)
    },
    setB ({ commit }, b) {
      commit('SET_B', b)
    }
  }
})

Home.vue

<script>
// @ is an alias to /src
import HelloWorld from '@/components/HelloWorld.vue'

export default {
  name: 'home',
  components: {
    HelloWorld
  },
  created: function () {
    this.$store.dispatch('setA', 'A')
  }
}
</script>

About.vue

<script>
export default {
  name: 'home',
  components: {
  },
  created: function () {
    this.$store.dispatch('setB', 'B')
  }
}
</script>

Console log on Components tab

log.js?1afd:24 [HMR] Waiting for update signal from WDS...
store.js?c0d6:22 A
store.js?c0d6:13 A
backend.js:1585  vue-devtools  Detected Vue v2.6.10 

Console log on Vuex tab

log.js?1afd:24 [HMR] Waiting for update signal from WDS...
store.js?c0d6:22 A
store.js?c0d6:13 A
backend.js:1585  vue-devtools  Detected Vue v2.6.10 

store.js?c0d6:13 null

backend.js:14674 Uncaught TypeError: Cannot set property 'a' of null
    at Store.SET_A (store.js?c0d6:14)
    at wrappedMutationHandler (vuex.esm.js?2ba1:725)
    at backend.js:14664
    at Array.forEach (<anonymous>)
    at VuexBackend.replayMutations (backend.js:14664)
    at VuexBackend.onInspectState (backend.js:14355)
    at Bridge.emit (backend.js:5472)
    at Bridge._emit (backend.js:5172)
    at backend.js:5097
    at Array.forEach (<anonymous>)
SET_A @ store.js?c0d6:14
wrappedMutationHandler @ vuex.esm.js?2ba1:725
(anonymous) @ backend.js:14664
replayMutations @ backend.js:14664
onInspectState @ backend.js:14355
emit @ backend.js:5472
_emit @ backend.js:5172
(anonymous) @ backend.js:5097
(anonymous) @ backend.js:5097
listener @ backend.js:2568
postMessage (async)
o @ proxy.js:1

store.js?c0d6:13 null

backend.js:14674 Uncaught TypeError: Cannot set property 'a' of null
    at Store.SET_A (store.js?c0d6:14)
    at wrappedMutationHandler (vuex.esm.js?2ba1:725)
    at backend.js:14664
    at Array.forEach (<anonymous>)
    at VuexBackend.replayMutations (backend.js:14664)
    at VuexBackend.onInspectState (backend.js:14355)
    at Bridge.emit (backend.js:5472)
    at Bridge._emit (backend.js:5172)
    at backend.js:5097
    at Array.forEach (<anonymous>)
SET_A @ store.js?c0d6:14
wrappedMutationHandler @ vuex.esm.js?2ba1:725
(anonymous) @ backend.js:14664
replayMutations @ backend.js:14664
onInspectState @ backend.js:14355
emit @ backend.js:5472
_emit @ backend.js:5172
(anonymous) @ backend.js:5097
(anonymous) @ backend.js:5097
listener @ backend.js:2568
postMessage (async)
o @ proxy.js:1

Answer 1

It was "New Vuex backend" in devtools settings.

I don`t know what is the catch, but disable this option solves my problem.