diegoveloper/flutter_stretchy_header issue repro code

import 'package:flutter/material.dart';
import 'package:stretchy_header/stretchy_header.dart';

class StretchyHeaderTest extends StatefulWidget {
  @override
  _StretchyHeaderTestState createState() => _StretchyHeaderTestState();
}

class _StretchyHeaderTestState extends State<StretchyHeaderTest> {

  int selectedAmount = 0;

  void selectAmount(int amount) {
    selectedAmount = amount;
  }

  List<Widget> getAmountWidgetsList() {
    List<Widget> list = new List();

    for (int i = 1; i < 8; i++) {
      var selected = (selectedAmount == i);
      var amountWidget = Padding(
        padding: EdgeInsets.symmetric(horizontal: 15.0, vertical: 22.5),
        child: Material(
          color: selected ? Colors.grey[100] : Colors.white,
          borderRadius: BorderRadius.circular(10.0),
          elevation: selected ? 15.0 : 3.0,
          child: new InkWell(
            borderRadius: BorderRadius.circular(10.0),
            onTap: () {
              setState(() {
                selectAmount(i);
              });
            },
            child: Padding(
              padding: EdgeInsets.symmetric(vertical: 5.0, horizontal: 20.0),
              child: Column(
                children: <Widget>[
                  Text("Size x$i",
                      style: TextStyle(fontSize: 15.0)),
                  SizedBox(height: 5.0),
                  Text("really cute",
                      style: TextStyle(
                        fontSize: 12.5,
                        color: Colors.grey,
                        //fontStyle: FontStyle.italic
                      )),
                  SizedBox(height: 5.0),
                  Text("${i * 75}\$",
                      style: TextStyle(fontSize: 20.0)),
                ],
              ),
            ),
          ),
        ),
      );

      list.add(amountWidget);
    }

    return list;
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: Text("Teddy Bear")),
      body: Container(
        child: StretchyHeader.singleChild(
          headerData: HeaderData(
              headerHeight: 250,
              header: Image(
                  image: NetworkImage("https://www.o2lifestyle.es/wp-content/uploads/2016/07/osito-ted.jpg"),
                  fit: BoxFit.cover),
              blurContent: false,
              blurColor: Color.fromRGBO(255, 255, 255, 0.1)),
          child: Column(
            children: <Widget>[
              SizedBox(
                height: 15,
              ),
              Text(
                "Teddy Bear",
                style: TextStyle(fontSize: 45),
              ),
              SingleChildScrollView(
                controller: ScrollController(keepScrollOffset: false),
                scrollDirection: Axis.horizontal,
                child: Padding(
                  padding: const EdgeInsets.only(
                      bottom: 15.0, left: 7.0, right: 7.0),
                  child: Row(
                    mainAxisAlignment: MainAxisAlignment.spaceEvenly,
                    children: getAmountWidgetsList(),
                  ),
                ),
              ),
              SizedBox(height: 15),
              RaisedButton(
                child: Text("Add to cart"),
                onPressed: () {},
              ),
              SizedBox(height: 15),
            ],
          ),
        ),
      )
    );
  }
}