NoSuchMethodError: '<Unexpected Null Value>' method not found

I'm trying to build a web app on Flutter. I was trying to add in some code for some buttons I found online but I'm getting this error and I have no idea why. The code seems to have no errors in itself, but when I run the program I get this error.

NoSuchMethodError: '<Unexpected Null Value>' method not found

The program is basically looks like an app bar: it shows a bunch of rectangular buttons that include Settings, Log out, etc. Before this, I successfully built the backdrop and a rectangle for the buttons to go placed. The buttons themselves are causing this error.

Any help would be much appreciated!

import 'package:firebase_auth/firebase_auth.dart';
import 'package:flutter/material.dart';
import 'package:flutter/rendering.dart';
import '../time/time_view.dart';
import 'package:provider/provider.dart';
import '../../../app/constants/strings.dart';
import '../../../app/services/firebase_auth_service.dart';
import 'package:flutter_web_firebase_googe_auth/size_config.dart';

String name = '';
final FirebaseAuth _auth = FirebaseAuth.instance;
getCurrentUserName() async {
  final user = await _auth.currentUser();
  if (user.isAnonymous) {
    name = 'Guest';
  } else {
    name = user.displayName;
  }
}

class HomeView extends StatelessWidget {
  const HomeView({Key key}) : super(key: key);
  @override
  Widget build(BuildContext context) {
    // variable size
    Size size = MediaQuery.of(context).size;

    getCurrentUserName();
    return Scaffold(
      backgroundColor: Colors.blue[200],
      body: Center(
        child: Column(
          children: <Widget>[
            Padding(
              padding: const EdgeInsets.all(16),
              child: Card(
                shape: RoundedRectangleBorder(
                    borderRadius: BorderRadius.circular(30.0)),
                elevation: 5.0,
                child: Container(
                  width: size.width,
                  height: size.height * .9,
                  child: Column(
                    children: <Widget>[
                      Padding(
                        padding: const EdgeInsets.all(20),
                        child: Text(
                          '\t\tHome Page \n Welcome',
                          style: Theme.of(context).textTheme.headline4,
                        ),
                      ),
                      const Spacer(),
                      SpecialOffers(),
                      RaisedButton(
                        onPressed: () {
                          Navigator.push(
                            context,
                            MaterialPageRoute(builder: (context) => TimeView()),
                          );
                        },
                        child: const Text(Strings.time),
                      ),
                      const Spacer(),
                      RaisedButton(
                        onPressed: () {
                          context.read<FirebaseAuthService>().signOut();
                        },
                        child: const Text(Strings.signOut),
                      ),
                      const Spacer(),
                    ],
                  ),
                ),
              ),
            ),
          ],
        ),
      ),
    );
  }
}

class SectionTitle extends StatelessWidget {
  const SectionTitle({
    Key key,
    @required this.title,
    @required this.press,
  }) : super(key: key);

  final String title;
  final GestureTapCallback press;

  @override
  Widget build(BuildContext context) {
    return Row(
      mainAxisAlignment: MainAxisAlignment.spaceBetween,
      children: [
        Text(
          title,
          style: TextStyle(
            fontSize: getProportionateScreenWidth(18),
            color: Colors.black,
          ),
        ),
        GestureDetector(
          onTap: press,
          child: Text(
            "See More",
            style: TextStyle(color: Color(0xFFBBBBBB)),
          ),
        ),
      ],
    );
  }
}

class SpecialOffers extends StatelessWidget {
  const SpecialOffers({
    Key key,
  }) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return Column(
      children: [
        Padding(
          padding:
              EdgeInsets.symmetric(horizontal: getProportionateScreenWidth(20)),
          child: SectionTitle(
            title: "Special for you",
            press: () {},
          ),
        ),
        SizedBox(height: getProportionateScreenWidth(20)),
        SingleChildScrollView(
          scrollDirection: Axis.horizontal,
          child: Row(
            children: [
              SpecialOfferCard(
                image: "assets/images/Image Banner 2.png",
                category: "Smartphone",
                numOfBrands: 18,
                press: () {},
              ),
              SpecialOfferCard(
                image: "assets/images/Image Banner 3.png",
                category: "Fashion",
                numOfBrands: 24,
                press: () {},
              ),
              SizedBox(width: getProportionateScreenWidth(20)),
            ],
          ),
        ),
      ],
    );
  }
}

class SpecialOfferCard extends StatelessWidget {
  const SpecialOfferCard({
    Key key,
    @required this.category,
    @required this.image,
    @required this.numOfBrands,
    @required this.press,
  }) : super(key: key);

  final String category, image;
  final int numOfBrands;
  final GestureTapCallback press;

  @override
  Widget build(BuildContext context) {
    return Padding(
      padding: EdgeInsets.only(left: getProportionateScreenWidth(20)),
      child: GestureDetector(
        onTap: press,
        child: SizedBox(
          width: getProportionateScreenWidth(242),
          height: getProportionateScreenWidth(100),
          child: ClipRRect(
            borderRadius: BorderRadius.circular(20),
            child: Stack(
              children: [
                Image.asset(
                  image,
                  fit: BoxFit.cover,
                ),
                Container(
                  decoration: BoxDecoration(
                    gradient: LinearGradient(
                      begin: Alignment.topCenter,
                      end: Alignment.bottomCenter,
                      colors: [
                        Color(0xFF343434).withOpacity(0.4),
                        Color(0xFF343434).withOpacity(0.15),
                      ],
                    ),
                  ),
                ),
                Padding(
                  padding: EdgeInsets.symmetric(
                    horizontal: getProportionateScreenWidth(15.0),
                    vertical: getProportionateScreenWidth(10),
                  ),
                  child: Text.rich(
                    TextSpan(
                      style: TextStyle(color: Colors.white),
                      children: [
                        TextSpan(
                          text: "$category\n",
                          style: TextStyle(
                            fontSize: getProportionateScreenWidth(18),
                            fontWeight: FontWeight.bold,
                          ),
                        ),
                        TextSpan(text: "$numOfBrands Brands")
                      ],
                    ),
                  ),
                ),
              ],
            ),
          ),
        ),
      ),
    );
  }
}