Panel doesn't appear on package version 2.0.0 or 2.0.0+1

Describe the bug The Sliding Up Panel doesn't appear when using sliding_up_panel version 2.0.0 or 2.0.0+1, but it does appear for version 1.0.2.

To Reproduce Run the provided sample. The Sliding Up Panel only appears on the older package version.

Expected behavior The Sliding Up Panel should appear when run on all package versions.

Screenshots App using package 1.0.2, panel appears:

App using package 2.0.0+1, no panel:

Additional context

To run package version 1.0.2 on the latest flutter, we you must use use the Flutter flag --no-sound-null-safety
Issue occurs on desktop Linux too
Even though the Panel doesn't appear, the onPanelSlide() callback does appear to be running at each frame when using PanelController.open() and PanelController.close

Smartphone (please complete the following information):

Device: Android emulator (Pixel 3 XL)
OS: Android 9
Flutter Version 2.2.2
sliding_up_panel Version 2.0.0+1

Sample main.dart

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

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  const MyApp({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(
          title: Text("SlidingUpPanelExample"),
        ),
        body: SlidingUpPanel(
          panel: Center(
            child: Text("This is the sliding Widget"),
          ),
          body: Center(
            child: Text("This is the Widget behind the sliding panel"),
          ),
        ),
      ),
    );
  }
}

Flutter doctor output:

[✓] Flutter (Channel stable, 2.2.2, on Linux, locale en_AU.UTF-8)
    • Flutter version 2.2.2 at /opt/flutter
    • Framework revision d79295af24 (3 weeks ago), 2021-06-11 08:56:01 -0700
    • Engine revision 91c9fc8fe0
    • Dart version 2.13.3

[!] Android toolchain - develop for Android devices (Android SDK version 30.0.3)
    • Android SDK at /home/jaween/Android/Sdk
    • Platform android-30, build-tools 30.0.3
    • Java binary at: /opt/android-studio/jre/bin/java
    • Java version OpenJDK Runtime Environment (build 11.0.8+0-b944-P17168821)
    ✗ Android license status unknown.
      Run `flutter doctor --android-licenses` to accept the SDK licenses.
      See https://flutter.dev/docs/get-started/install/linux#android-setup for
      more details.

[✓] Chrome - develop for the web
    • CHROME_EXECUTABLE = chromium

[✓] Linux toolchain - develop for Linux desktop
    • clang version 12.0.0
    • cmake version 3.20.3
    • ninja version 1.10.2
    • pkg-config version 1.7.3

[✓] Android Studio (version 4.2)
    • Android Studio at /opt/android-studio
    • Flutter plugin version 56.0.2
    • Dart plugin version 202.8488
    • Java version OpenJDK Runtime Environment (build 11.0.8+0-b944-P17168821)

[✓] Connected device (3 available)
    • Android SDK built for x86 64 (mobile) • emulator-5554 • android-x64    •
      Android 9 (API 28) (emulator)
    • Linux (desktop)                       • linux         • linux-x64      •
      Linux
    • Chrome (web)                          • chrome        • web-javascript •
      Chromium 91.0.4472.114 Arch Linux

! Doctor found issues in 1 category.

akshathjain / sliding_up_panel

Panel doesn't appear on package version 2.0.0 or 2.0.0+1 #259