An example of my broken shared preference flutter slider

fontsize.dart

import 'package:flutter/material.dart';
import 'package:flutter_html/style.dart';
import 'package:flutter_html/flutter_html.dart';
import 'package:shared_preferences/shared_preferences.dart';


class FontSizer extends StatefulWidget {
  @override
  _FontSizerState createState() => new _FontSizerState();
}

class _FontSizerState extends State<FontSizer> {
  Future<SharedPreferences> _prefs = SharedPreferences.getInstance();
  Future<double> fontSizeValue;
  @override
  void initState() {
    super.initState();
    _loadFontSize();
  }

  _loadFontSize() async {
    final prefs = await _prefs;
    final double fs = prefs.getDouble('fontSize') ?? 18.0;
    setState(() {
      fontSizeValue = prefs.setDouble("fontSize", fs).then((bool success) {
        return fs;
      });
    });
    print('fontsize: $fs');
  }

  _saveFontSize(newFontSize) async {
    // obtain shared preferences
    final prefs = await _prefs;

    // set value
    prefs.setDouble('fontSize', newFontSize);
    print("new fontsize set to $newFontSize");
  }

  Widget _buildFontSizer() {
    return FutureBuilder(
      future: fontSizeValue,
      builder: (BuildContext context, AsyncSnapshot snapshot) {
        switch (snapshot.connectionState) {
          case ConnectionState.waiting:
            return const CircularProgressIndicator();
          default:
            if (snapshot.hasError) {
              return Text('Error: ${snapshot.error}');
            } else {
              print('Snapshot fontsize ${snapshot.data}');
              double x = snapshot.data;
              return Column(
                children: <Widget>[ Padding(
                  padding: const EdgeInsets.only(top: 48.0),
                  child: SliderTheme(
                    data: SliderTheme.of(context).copyWith(
                      activeTrackColor: Colors.red[700],
                      inactiveTrackColor: Colors.red[100],
                      trackShape: RoundedRectSliderTrackShape(),
                      trackHeight: 4.0,
                      thumbShape: RoundSliderThumbShape(
                          enabledThumbRadius: 12.0),
                      thumbColor: Colors.redAccent,
                      overlayColor: Colors.red.withAlpha(32),
                      overlayShape: RoundSliderOverlayShape(
                          overlayRadius: 28.0),
                      tickMarkShape: RoundSliderTickMarkShape(),
                      activeTickMarkColor: Colors.red[700],
                      inactiveTickMarkColor: Colors.red[100],
                      valueIndicatorShape: PaddleSliderValueIndicatorShape(),
                      valueIndicatorColor: Colors.redAccent,
                      valueIndicatorTextStyle: TextStyle(
                        color: Colors.white,
                      ),
                    ),
                    child: Slider(
                      value: x,
                      min: 16,
                      max: 30,
                      divisions: 14,
                      label: '$x',
                      onChanged: (value) {
                        setState(() {
                          x = value;
                        },
                        );
                        print(value);
                      },
                      onChangeEnd: (value) {
                        setState(() {
                          (value);
                        },
                        );
                        print(value);
                      },

                    ),
                  ),
                ),
                  Html(
                      data: "<font color=\"#0B6138\"><b>1</b></font><br>\nAll creatures of our God and King,<br>\nLift up your voice with us and sing:<br>\nAlleluia! Alleluia!<br>\nO burning sun with golden beam<br>\nAnd silver moon with softer gleam:<br>\n<br>\n<i><b><font color=\"#CD9B1D\">CHORUS:</font></b><br>\nOh, praise Him! Oh, praise Him!<br>\nAlleluia, alleluia, alleluia!<br>\n</i><br>\n<font color=\"#0B6138\"><b>2</b></font><br>\nO rushing wind and breezes soft,<br>\nO clouds that ride the winds aloft:<br>\nOh, praise Him! Alleluia!<br>\nO rising morn, in praise rejoice,<br>\nO lights of evening, find a voice.<br>\n<br>\n",
                      style: {
                        "html": Style(color: Colors.black,
                            fontSize: FontSize(20.0))
                      }
                  )
                ],
              );
            }
        }
      },
    );
  }

  @override
  Widget build(BuildContext context) {
    return new Scaffold(
      appBar:AppBar(
        title: Text("Font Sizer")
      ),
      body: Padding(
        padding: const EdgeInsets.all(10.0),
        child: SingleChildScrollView(
            child: _buildFontSizer()
        ),
      ),
    );
  }
}