Selecting all the text and then scrolling will cause an error: Null check operator used on a null value

[wilinz]

Describe the bug
Selecting all the text and then scrolling will cause an error: Null check operator used on a null value

To Reproduce
Steps to reproduce the behavior:

1. long press
2. select all
3. Scroll
4. See error

Expected behavior
A clear and concise description of what you expected to happen.

Screenshots

Environment(please complete the following information):
Please use flutter doctor -v to show your flutter environment, and add your markdown_widget version too

Platform
Android

Source data
https://github.com/wilinz/flutter_markdown_demo

[wilinz]

Screenrecorder-2023-07-04-19-57-06-130.mp4

[wilinz]

======== Exception caught by widgets library =======================================================
The following _TypeError was thrown building _OverlayEntryWidget-[LabeledGlobalKey<_OverlayEntryWidgetState>#7f1ea](dirty, state: _OverlayEntryWidgetState#d9246):
Null check operator used on a null value
The relevant error-causing widget was:
MaterialApp MaterialApp:file:https:///C:/Users/wilinz/Desktop/gitee/flutter_markdown/lib/main.dart:18:12
When the exception was thrown, this was the stack:
0 SelectableRegionState.startGlyphHeight (package:flutter/src/widgets/selectable_region.dart:1028:56)
1 SelectableRegionState.contextMenuAnchors (package:flutter/src/widgets/selectable_region.dart:912:25)
2 new AdaptiveTextSelectionToolbar.selectableRegion (package:flutter/src/material/adaptive_text_selection_toolbar.dart:169:40)
3 SelectionArea._defaultContextMenuBuilder (package:flutter/src/material/selection_area.dart:96:41)
4 SelectableRegionState._showToolbar. (package:flutter/src/widgets/selectable_region.dart:769:42)
5 SelectionOverlay.showToolbar. (package:flutter/src/widgets/text_selection.dart:1347:36)
6 ContextMenuController.show. (package:flutter/src/widgets/context_menu_controller.dart:65:54)
7 _OverlayEntryWidgetState.build (package:flutter/src/widgets/overlay.dart:345:36)
8 StatefulElement.build (package:flutter/src/widgets/framework.dart:5198:27)
9 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:5086:15)
10 StatefulElement.performRebuild (package:flutter/src/widgets/framework.dart:5251:11)
11 Element.rebuild (package:flutter/src/widgets/framework.dart:4805:7)
12 BuildOwner.buildScope (package:flutter/src/widgets/framework.dart:2780:19)
13 WidgetsBinding.drawFrame (package:flutter/src/widgets/binding.dart:903:21)
14 RendererBinding._handlePersistentFrameCallback (package:flutter/src/rendering/binding.dart:358:5)
15 SchedulerBinding._invokeFrameCallback (package:flutter/src/scheduler/binding.dart:1284:15)
16 SchedulerBinding.handleDrawFrame (package:flutter/src/scheduler/binding.dart:1214:9)
17 SchedulerBinding._handleDrawFrame (package:flutter/src/scheduler/binding.dart:1072:5)
18 _invoke (dart:ui/hooks.dart:142:13)
19 PlatformDispatcher._drawFrame (dart:ui/platform_dispatcher.dart:359:5)
20 _drawFrame (dart:ui/hooks.dart:112:31)

Lost connection to device.

[asjqkkkk]

Hi @wilinz , I thinks this is not a bug of markdown_widget, you should remove the Expanded inside the Padding.

[Easy-AI-1688]

This is a bug in Flutter. Please upgrade Flutter to the latest version and this issue will be resolved.