Vuex action does not work properly in a vue mounted hook

I'm building a small e-commerce store with an admin panel for myself. I use Firebase firestore as my backend to store all the user's data.
I have a root 'users' collection with a document for every single registered user and everything else each user has is branching out of the user doc.

Here are firestore commands i perform so you understand the structure better.

db.collection('users').doc(userId).collection('categories').doc(subCategoryId)... db.collection('users').doc(userId).collection('subcategories').doc(subCategoryId)...

I use Vuex so every time i need to change something on my firestore (update a product category, remove a category etc.), i dispatch an appropriate action. The first thing any of those actions does is to go ahead and dispatch another action from auth.js that gets the userId.

The problem is that if the action in question should run in a mounted() lifecycle hook, then it fails to grab the userId.
In EditCategory.vue updateCategory action works perfectly well because SubmitHandler() is triggered on click event but in Categories.vue the fetchCategories does not work and spit out an error:

[Vue warn]: Error in mounted hook (Promise/async): "FirebaseError: [code=invalid-argument]: Function CollectionReference.doc() requires its first argument to be of type non-empty string, but it was: null"

Function CollectionReference.doc() requires its first argument to be of type non-empty string, but it was: null

Which, as far as i understand it, basically tells me that fetchCategories() action's firestore query could not be performed because the userId was not recieved.

After two days of moving stuff around i noticed that errors only occur if i refresh the page. If i switch to other tab and back on without refreshing, then fetchCategories() from Categories.vue mounted() hook works. Placing the code in to created() hook gives the same result.

I think that there is some fundamental thing i am missing about asynchronous code and lifecycle hooks.

Categories.vue component

 <template>
    <div class="category-main">
      <section>
        <div class="section-cols" v-if="!loading">
          <EditCategory 
            v-on:updatedCategory="updatedCategory"
            v-bind:categories="categories"
            v-bind:key="categories.length + updateCount"
          />
        </div>
      </section>
    </div>
    </template>
    
    <script>
    import EditCategory from '@/components/admin/EditCategory.vue'
    
    export default {
      name: 'AdminCategories',
      components: {
        EditCategory,
      },
      data: () => ({
        updateCount: 0, 
        loading: true, 
        categories: [],
      }),
      async mounted() {
        this.categories = await this.$store.dispatch('fetchCategories');// FAILS!
        this.loading = false;
      },
      methods: {
        addNewCategory(category) {
          this.categories.push(category);
        },
        updatedCategory(category) {
          const catIndex = this.categories.findIndex(c => c.id === category.id);
          this.categories[catIndex].title = category.title;
          this.categories[catIndex].path = category.path;
          this.updateCount++;
        }
      }
    
    }
    </script>

category.js store file

import firebase, { firestore } from "firebase/app";
import db from '../../fb';

export default {
  actions: {
    async getUserId() {
      const user = firebase.auth().currentUser;
      return user ? user.uid : null;
    },
    
    export default {
      state: {
        test: 10,
        categories: [],
        subCategories: [],
        currentCategory: '',
      },
      mutations: {
        setCategories(state, payload){
          state.categories = payload;
        },
      },
      actions: {
    
        async fetchCategories({commit, dispatch}) {
          try {
            const userId = await dispatch('getUserId');
            const categoryArr = [];
            await db.collection('users').doc(userId).collection('categories').get().then((querySnapshot) => {
              querySnapshot.forEach((doc) => {
                 categoryArr.push({ id: doc.id, ...doc.data() })
              });
           })
            commit('setCategories', categoryArr);
            return categoryArr;
          } catch (err) { throw err; }
        },
    
    
        async updateCategory({commit, dispatch}, {title, path, id}) {
          try {
            const userId = await dispatch('getUserId');
            console.log('[category.js] updateCategory', userId);
            await db.collection('users').doc(userId).collection('categories').doc(id).update({
              title, 
              path
            })
            commit('rememberCurrentCategory', id);
            return;
          } catch (err) {throw err;}
        }
      },
    }

auth.js store file

import firebase, { firestore } from "firebase/app";
import db from '../../fb';


export default {
  actions: {

    ...async login(), async register(), async logout()

    async getUserId() {
      const user = firebase.auth().currentUser;
      return user ? user.uid : null;
    },
  },
}

index.js store file

import Vue from 'vue'
import Vuex from 'vuex'
import auth from './auth'
import products from './products'
import info from './info'
import category from './category'
Vue.use(Vuex)

export default new Vuex.Store({
  modules: {
    auth, products, info, category,
  }
})

EditCategory.vue

export default {
  name: 'EditCategory',
  data: () => ({
    select: null,
    title: '',
    path: '',
    current: null
  }),
  props: {
    categories: {
      type: Array,
      required: true
    }
  },
  methods: {
    async submitHandler() {
      if (this.$v.invalid){
          this.$v.$touch()
          return;
      }
      try {
        const categoryData = {
            id : this.current,
            title: this.title,
            path: this.path
        };
        await this.$store.dispatch('updateCategory', categoryData);// WORKS!
        this.$emit('updatedCategory', categoryData);
      } catch (err) { throw err; }
    },
  },
  //takes current category id from store getter
  computed: {
    categoryFromState() {
      return this.$store.state.currentCategory;
    }
  },
  created() {
    console.log('[EditCategory.vue'], currentCategory);
  },
  mounted(){
    this.select = M.FormSelect.init(this.$refs.select);
    M.updateTextFields();
  },
  destroyed() {
    if (this.select && this.select.destroy) {
        this.select.destroy;
    }
  }
}
</script>

First of all, it's just a small detail, but you don't need need to make your 'getUserId' action async, since it does not use the 'await' keyword. So can simplify this :

async getUserId() {
  const user = firebase.auth().currentUser;
  return user ? user.uid : null;
}

const userId = await dispatch('getUserId')

into this :

getUserId() {
  const user = firebase.auth().currentUser;
  return user ? user.uid : null;
}

const userId = dispatch('getUserId')

Coming back to your id that seems to be undefined, the problem here is that your 'mounted' event is probably triggered before the 'login' can be completed.

How to solve this case ? Actually, there are a lot of different ways to approch this. What I suggest in your case is to use a middleware (or a 'route guard'). This guard can make you are verified user before accessing some routes (and eventually restrict the access or redirect depending on the user privileges). In this way, you can make sure that your user is defined before accessing the route.

This video is 4 years old so it is not up to date with the last versions of Firebas. But I suggest The Net Ninja tutorial about Vue Route Guards with Firebase if you want to learn more about this topic.