🐛 The PaneItemExpander doesn't work normal

[H2Sxxa]

Describe the bug

The PaneItemExpander doesn't work normal.

════════ Exception caught by widgets library ═══════════════════════════════════
The following _TypeError was thrown building _PaneItemExpander-[LabeledGlobalKey<__PaneItemExpanderState>#18916](dirty, dependencies: [_FluentTheme, _InheritedNavigationView], state: __PaneItemExpanderState#fd9cf):
Null check operator used on a null value

The relevant error-causing widget was
_PaneItemExpander-[LabeledGlobalKey<__PaneItemExpanderState>#18916]
pane_items.dart:629
When the exception was thrown, this was the stack
#0      __PaneItemExpanderState.build
pane_items.dart:807
#1      StatefulElement.build
framework.dart:5409
#2      ComponentElement.performRebuild
framework.dart:5297
#3      StatefulElement.performRebuild
framework.dart:5462
#4      Element.rebuild
framework.dart:5016
#5      ComponentElement._firstBuild
framework.dart:5279
#6      StatefulElement._firstBuild
framework.dart:5453
#7      ComponentElement.mount
framework.dart:5273
...     Normal element mounting (47 frames)
#54     Element.inflateWidget
framework.dart:4182
#55     Element.updateChild
framework.dart:3707
#56     SliverMultiBoxAdaptorElement.updateChild
sliver.dart:857
#57     SliverMultiBoxAdaptorElement.createChild.<anonymous closure>
sliver.dart:842

To Reproduce

import 'package:fluent_ui/fluent_ui.dart';

void main() {
  runApp(const Demo());
}

class Demo extends StatelessWidget {
  const Demo({super.key});

  @override
  Widget build(BuildContext context) {
    return FluentApp(
        title: 'Flutter Demo',
        home: NavigationView(
          pane: NavigationPane(items: [
            PaneItem(
                title: const Text("Test"),
                icon: const Icon(FluentIcons.accept),
                body: ScaffoldPage.scrollable(
                    children: const [Text("Hello world")])),
            PaneItemSeparator(),
            PaneItemExpander(
                icon: const Icon(FluentIcons.toolbox),
                items: [],
                body: const Text("Expander"))
          ]),
        ));
  }
}

Expected behavior

The Expander should not crash here.

Screenshots

Additional context

Version

fluent_ui: ^4.7.7

Flutter Docker

Doctor summary (to see all details, run flutter doctor -v):
[√] Flutter (Channel stable, 3.13.9, on Microsoft Windows [版本 10.0.22635.2700], locale zh-CN)
[√] Windows Version (Installed version of Windows is version 10 or higher)
[X] Android toolchain - develop for Android devices
    X Unable to locate Android SDK.
      Install Android Studio from: https://developer.android.com/studio/index.html
      On first launch it will assist you in installing the Android SDK components.
      (or visit https://flutter.dev/docs/get-started/install/windows#android-setup for detailed instructions).
      If the Android SDK has been installed to a custom location, please use
      `flutter config --android-sdk` to update to that location.

[X] Chrome - develop for the web (Cannot find Chrome executable at .\Google\Chrome\Application\chrome.exe)
    ! Cannot find Chrome. Try setting CHROME_EXECUTABLE to a Chrome executable.
[√] Visual Studio - develop Windows apps (Visual Studio Community 2022 17.6.3)
[!] Android Studio (not installed)
[√] VS Code (version 1.84.1)
[√] Connected device (2 available)
[√] Network resources

! Doctor found issues in 3 categories.

[H2Sxxa]

Hello! I solved now

PaneItemExpander must have a not empty item list and the selected of NavigationPane must be not null.

I think there should have a more clear exception info, like assert(body.pane!.selected != null, "The selected of NavigationPane can not be null!"); in the build Function.

Code

import 'package:fluent_ui/fluent_ui.dart';

void main() {
  runApp(const Demo());
}

class Demo extends StatelessWidget {
  const Demo({super.key});

  @override
  Widget build(BuildContext context) {
    return FluentApp(
        title: 'Flutter Demo',
        home: NavigationView(
          pane: NavigationPane(selected: 0, items: [
            PaneItem(
                title: const Text("Test"),
                icon: const Icon(FluentIcons.accept),
                body: ScaffoldPage.scrollable(
                    children: const [Text("Hello world")])),
            PaneItemSeparator(),
            PaneItemExpander(
                icon: const Icon(FluentIcons.toolbox),
                items: [
                  PaneItem(
                      title: const Text("Test"),
                      icon: const Icon(FluentIcons.accept),
                      body: ScaffoldPage.scrollable(
                          children: const [Text("Hello world 2")])),
                ],
                body: const Text("Expander"))
          ]),
        ));
  }
}