Not able to fetch particular user document by user id in flutter app

Question

I want to retrieve a particular user document by its id from the collection users. When I directly pass the particular user id, I get the data. But when I pass it using variable it shows null.

My code is as follows:

import 'package:cloud_firestore/cloud_firestore.dart';
import 'package:flutter/material.dart';
import 'package:firebase_auth/firebase_auth.dart';
import '../services/crud.dart';

class test extends StatefulWidget {
  @override
  _testState createState() => _testState();

}
class _testState extends State<test> {

  String userID="";
  @override
  void initState() {
    super.initState();
    ///get current user and assign his id
    FirebaseAuth.instance.currentUser().then((FirebaseUser user) {
      setState(() {
        userID = user.uid;
        print(userID);
      });
    });
  }
  @override
  Widget build(BuildContext context) {
    return SingleChildScrollView(
      child: Column(
        children: <Widget>[
          StreamBuilder(
                  stream: Firestore.instance.collection('users').document(userID).snapshots(),
                  builder: (context,snapshot){
                    if (!snapshot.hasData) return const Text("Loading...");
                    else return Container(
                      child: Column(
                        children: <Widget>[
                          Text(snapshot.data["name"]),
                          Text(snapshot.data["email"]),
                          Text(snapshot.data["phone"].toString()),
                        ],
                      ),
                    );
                  },
                )
        ],
      )
    );
  }
}

When I'm using the following line of code with specifying uid it shows the result :

stream: Firestore.instance.collection('users').document('d6DshRomJMkIe9mAARAi').snapshots(),

But it does not work when I pass userID inside the document(). Even though userID contains the actual id of the logged-in user.

stream: Firestore.instance.collection('users').document(userID).snapshots(),

The error says :

NoSuchMethodError: The method '[]' was called on null.
Receiver: null
Tried calling []("name")

This is the structure of my database.

This is the error that I get on my app screen.

Solution

When using userID is doesn't work because currentUser() is asynchronous and the StreamBuilder is being called even before getting the userId therefore try the following:

   Stream<DocumentSnapshot> getData()async*{
     FirebaseUser user = await FirebaseAuth.instance.currentUser();
     yield* Firestore.instance.collection('users').document(user.uid).snapshots();
  }

Create a method that returns a Stream and then inside the StreamBuilder do the following:

children: <Widget>[
          StreamBuilder(
                  stream: getData(),
                  builder: (context,snapshot){
                    if (!snapshot.hasData) return const Text("Loading...");
                    else if(snapshot.hasData){
                    return Container(
                      child: Column(
                        children: <Widget>[
                          Text(snapshot.data["name"]),
                          Text(snapshot.data["email"]),
                          Text(snapshot.data["phone"].toString()),
                        ],
                      ),
                    );
                   },
                return CircularProgressIndicator();
                  },
                )
        ],