[Bug] Null reference exception in Shell when switching between tabs

[ChristopherStephan]

Description

We get a null reference exception in our project as well as in the Xaminals app when switching between shell tabs:

Steps to Reproduce

1. Download Xaminals project
2. Update Xamarin.Forms to version 4.8.0.1269
3. Deploy Xaminals app on iOS 14.3
4. Switch between the tabs
5. App crashes with error:

 System.NullReferenceException: Object reference not set to an instance of an object
  at Xamarin.Forms.Shell.GetNavBarIsVisible (Xamarin.Forms.BindableObject obj) [0x00000] in D:\a\1\s\Xamarin.Forms.Core\Shell\Shell.cs:104
  at Xamarin.Forms.Platform.iOS.ShellSectionRenderer+NavDelegate.WillShowViewController (UIKit.UINavigationController navigationController, UIKit.UIViewController viewController, System.Boolean animated) [0x00028] in D:\a\1\s\Xamarin.Forms.Platform.iOS\Renderers\ShellSectionRenderer.cs:628
  at at (wrapper managed-to-native) UIKit.UIApplication.UIApplicationMain(int,string[],intptr,intptr)
  at UIKit.UIApplication.Main (System.String[] args, System.IntPtr principal, System.IntPtr delegate) [0x00005] in /Library/Frameworks/Xamarin.iOS.framework/Versions/14.8.0.3/src/Xamarin.iOS/UIKit/UIApplication.cs:86
  at UIKit.UIApplication.Main (System.String[] args, System.String principalClassName, System.String delegateClassName) [0x0000e] in /Library/Frameworks/Xamarin.iOS.framework/Versions/14.8.0.3/src/Xamarin.iOS/UIKit/UIApplication.cs:65
  at Xaminals.iOS.Application.Main (System.String[] args) [0x00001] in /Users/destec2/Documents/xaminals/xamarin-forms-samples/UserInterface/Xaminals/Xaminals.iOS/Main.cs:12

Expected Behavior

App does not crash when navigating between tabs

Actual Behavior

App crashes when switching between the tabs

Basic Information

• on iOS 13.7 the crash is not reproducible
• iOS: 14.3
• Related issue: [Bug] Shell - Go back two pages crashes the app with a NullReferenceException #6484

Environment

=== Visual Studio Community 2019 for Mac ===

Version 8.8.6 (build 15)
Installation UUID: f2862ade-ea94-46dc-80e1-8b8e0ee2217c
	GTK+ 2.24.23 (Raleigh theme)
	Xamarin.Mac 6.18.0.23 (d16-6 / 088c73638)

	Package version: 612000113

=== Mono Framework MDK ===

Runtime:
	Mono 6.12.0.113 (2020-02/4fdfb5b1fd5) (64-bit)
	Package version: 612000113

=== Roslyn (Language Service) ===

3.8.0-5.20519.18+4c195c3ac1974edcefa76774d7a59a2350ec55fa

=== NuGet ===

Version: 5.8.0.6860

=== .NET Core SDK ===

SDK: /usr/local/share/dotnet/sdk/5.0.102/Sdks
SDK Versions:
	5.0.102
	3.1.201
	2.1.802
MSBuild SDKs: /Applications/Visual Studio.app/Contents/Resources/lib/monodevelop/bin/MSBuild/Current/bin/Sdks

=== .NET Core Runtime ===

Runtime: /usr/local/share/dotnet/dotnet
Runtime Versions:
	5.0.2
	3.1.8
	3.1.3
	2.1.23
	2.1.22
	2.1.13

=== .NET Core 3.1 SDK ===

SDK: 3.1.201

=== Xamarin.Profiler ===

Version: 1.6.12.29
Location: /Applications/Xamarin Profiler.app/Contents/MacOS/Xamarin Profiler

=== Updater ===

Version: 11

=== Apple Developer Tools ===

Xcode 12.3 (17715)
Build 12C33

=== Xamarin.Mac ===

Version: 7.2.0.3 (Visual Studio Community)
Hash: c51fabee8
Branch: xcode12.3
Build date: 2020-12-10 21:05:04-0500

=== Xamarin.iOS ===

Version: 14.8.0.3 (Visual Studio Community)
Hash: c51fabee8
Branch: xcode12.3
Build date: 2020-12-10 21:05:05-0500

=== Xamarin.Android ===

Version: 11.1.0.26 (Visual Studio Community)
Commit: xamarin-android/d16-8/a36ce73
Android SDK: /Users/Shared/Development/Android/sdk
	Supported Android versions:
		8.0 (API level 26)
		8.1 (API level 27)

SDK Tools Version: 26.1.1
SDK Platform Tools Version: 29.0.4
SDK Build Tools Version: 29.0.2

Build Information: 
Mono: 5e9cb6d
Java.Interop: xamarin/java.interop/d16-8@79d9533
ProGuard: Guardsquare/proguard/proguard6.2.2@ebe9000
SQLite: xamarin/sqlite/3.32.1@1a3276b
Xamarin.Android Tools: xamarin/xamarin-android-tools/d16-8@2fb1cbc

=== Microsoft OpenJDK for Mobile ===

Java SDK: /Users/Shared/Development/jdk/microsoft_dist_openjdk_8.0.25
1.8.0-25
Android Designer EPL code available here:
https://github.com/xamarin/AndroidDesigner.EPL

=== Android SDK Manager ===

Version: 16.8.0.32
Hash: 01a7774
Branch: remotes/origin/d16-8
Build date: 2021-01-14 00:34:58 UTC

=== Android Device Manager ===

Version: 16.8.0.46
Hash: 0a81419
Branch: remotes/origin/d16-8
Build date: 2021-01-14 00:35:22 UTC

=== Xamarin Designer ===

Version: 16.8.0.510
Hash: 44e3f3ce9
Branch: remotes/origin/d16-8
Build date: 2020-12-10 00:06:14 UTC

=== Build Information ===

Release ID: 808060015
Git revision: d34d29b4643a130479d762e4d2b5750e6462fde7
Build date: 2021-01-15 08:35:04-05
Build branch: release-8.8
Xamarin extensions: d34d29b4643a130479d762e4d2b5750e6462fde7

=== Operating System ===

Mac OS X 10.15.7
Darwin 19.6.0 Darwin Kernel Version 19.6.0
    Mon Aug 31 22:12:52 PDT 2020
    root:xnu-6153.141.2~1/RELEASE_X86_64 x86_64

=== Enabled user installed extensions ===

DeepClean 1.2.5

[rachelkang]

Hi, @ChristopherStephan - thanks for submitting this issue. I unfortunately wasn't able to reproduce the behavior you're describing on the version of iOS and XF you specified... Can you try upgrading to 5.0 and letting me know if that changes anything? If not, could you let me know which device/emulator(s) are you experiencing this on? Thanks :)

[ChristopherStephan]

I was experiencing this issue on a iPhone 8 Plus simulator. Sorry, cannot re-test it with XF 5 now. Will try to find some time later.

[ChristopherStephan]

I could not reproduce the issue with Xamarin.Forms 5. This could be a false positive result though since the bug is not always reproducible.

[InquisitorJax]

@jsuarezruiz
Just a heads up here - we're still getting reports of this crash on App Center, even after upgrading Forms >= 2196
Stack trace as above:
Shell.GetNavBarIsVisible (Xamarin.Forms.BindableObject obj)
ShellSectionRenderer+NavDelegate.WillShowViewController (UIKit.UINavigationController navigationController, UIKit.UIViewController viewController, System.Boolean animated)
(wrapper managed-to-native) ObjCRuntime.Messaging.objc_msgSendSuper(intptr,intptr,bool)
UIViewController.ViewWillAppear (System.Boolean animated)
ShellSectionRenderer.ViewWillAppear (System.Boolean animated)
(wrapper managed-to-native) ObjCRuntime.Messaging.objc_msgSendSuper(intptr,intptr,intptr)