Failed to get document because the client is offline.

I am trying to develop an android app using nativescript and firebase as backend.To use Firebase I am using this plugin. I am trying to use Firestore for my app .I get this error each time I try to start the app.When I used realtime database ,I was easily able to access all the data, so this is not a network or configuration issue. Below is the code I am using to connet firebase and use firestore.

 firebase.initializeApp({}).then((data)=>{
           console.log(data) //it prints the databaseURL and it is correct.
        });
        var db = firebase.firestore();
        db.collection("users").doc('<docKey>').get().then((doc) => {
            if (doc.exists) {
                this.data =doc.data();
            } else{
                this.data ="No such document!";
            }
        }).catch((error) => {
            console.log(error);
        });

Error : com.google.firebase.firestore.FirebaseFirestoreException: Failed to get document because the client is offline.