Search in Firestore flutter

I am trying to make the SearchBar for an UI and when I try to search nothing it's happening. I put some Cases like Error or nothing found or done and the UI remains with a white screen when I search.What is wrong with my code?

import 'package:cloud_firestore/cloud_firestore.dart';
import 'package:flappy_search_bar/flappy_search_bar.dart';
import 'package:flutter/material.dart';

class Searchbar extends StatefulWidget {
  @override
  SearchBarState createState() => SearchBarState();
}

class SearchBarState extends State<Searchbar> {
  TextEditingController textEditingController = TextEditingController();
  final database = Firestore.instance;
  String searchString;

  @override
  Widget build(BuildContext context) {

    return Scaffold(
        body: Container(
            child: Column(
      children: <Widget>[
        Expanded(
          child: Column(
            children: <Widget>[
              Padding(
                padding: const EdgeInsets.all(15.0),
                child: Container(
                  child: TextField(
                    onChanged: (val) {
                      setState(() {
                        searchString = val.toLowerCase();
                      });
                    },
                    controller: textEditingController,
                    decoration: InputDecoration(
                        suffixIcon: IconButton(
                            icon: Icon(Icons.clear),
                            onPressed: () => textEditingController.clear()),
                        hintText: " Search name here ",
                        hintStyle: TextStyle(
                            fontFamily: 'Antra', color: Colors.blueGrey)),
                  ),
                ),
              ),
              Expanded(
                  child: StreamBuilder<QuerySnapshot>(
                stream: (searchString == null || searchString.trim() == '')
                    ? Firestore.instance.collection('Users').snapshots()
                    : Firestore.instance
                        .collection('Users')
                        .where('name', arrayContains: searchString)
                        .snapshots(),
                builder: (context, snapshot) {
                  if (snapshot.hasError) {
                    return Text('We got an error ${snapshot.error}');
                  }
                  switch (snapshot.connectionState) {
                    case ConnectionState.waiting:
                      return SizedBox(
                        child: Center(
                            // child: Lottie.asset('animations/loading2.json'),
                            ),
                      );
                    case ConnectionState.none:
                      return Text('Oops no data present');

                    case ConnectionState.done:
                      return Text('We are done');

                    default:
                      return new ListView(
                          children: snapshot.data.documents
                              .map((DocumentSnapshot document) {
                        return new ListTile(
                          title: Text(document['Users']),
                        );
                      }).toList());
                  }
                },
              ))
            ],
          ),
        )
      ],
    )));
  }
}

and this is the Firestore : 1 I want to search users by name and I added this to my code but I don't know why it doesn't work