title: $fireDb undefined in Nuxt Fire / Firebase Vuex
body: <p>I'm using Nuxt JS 2.9.2 to build a Javascript application. It features a Firebase integration via the Nuxt Fire module, and Vuex to handle state management and authentication.</p>
  
  <p>I have the project set up, and I'm able to create users, log them in and log them out. </p>
  
  <p>I'm trying to now set some data along with the user ID in Firebase, and am getting an <code>$fireDb</code> undefined error, even though the user is still being created (without any information set):</p>
  
  <pre><code>function createNewAccount (user) {
    const databaseRef = this.$fireDb.ref(`accounts/${user.uid}`)
    return databaseRef.set({
      displayName: 'Test Display Name', // use part of the email as a username
      email: 'My Test Email',
      image: 'Some Image' // supply a default profile image for all users
    })
  }
  
  export const actions = {
  
    /*
     * Create a user
     */
    createUser ({commit}, payload) {
      this.$fireAuth.createUserWithEmailAndPassword(payload.email, payload.password).then(function({user}) {
        commit('localStorage/setUser', { email: payload.email }, { root: true })
        createNewAccount(user)
      }).catch(function(error) {
        console.log('error registering' + error)
      });
    }
  }
  </code></pre>
  
  <p>I'm trying to figure out why <code>this.$fireDb</code> is undefined? I've tried following the documentation, but this part seems broken?</p>
  
  <p><a href="https://github.com/lupas/nuxt-fire#usage" rel="nofollow noreferrer">https://github.com/lupas/nuxt-fire#usage</a></p>