[shell] [windows] fix NRE when clearing shell items

[jonathanpeppers]

Fixes: #13775

You can cause MAUI to throw an NRE by doing:

1. Debug/run, have XAML hot reload enabled

2. Comment out the ShellContent in AppShell.xaml, hot reload

3. Uncomment the ShellContent in AppShell.xaml, hot reload

Crashes with:

System.NullReferenceException: Object reference not set to an instance of an object.
at Microsoft.Maui.Controls.Handlers.ShellSectionHandler.SetVirtualView(IElement view)
at Microsoft.Maui.Controls.Handlers.ShellItemHandler.UpdateCurrentItem()
at Microsoft.Maui.Controls.Handlers.ShellItemHandler.MapCurrentItem(ShellItemHandler handler, ShellItem item)
at Microsoft.Maui.PropertyMapper`2.<>c__DisplayClass5_0.<Add>b__0(IElementHandler h, IElement v)
at Microsoft.Maui.PropertyMapper.UpdatePropertyCore(String key, IElementHandler viewHandler, IElement virtualView)
at Microsoft.Maui.PropertyMapper.UpdateProperties(IElementHandler viewHandler, IElement virtualView)
at Microsoft.Maui.Handlers.ElementHandler.SetVirtualView(IElement view)
at Microsoft.Maui.Controls.Handlers.ShellItemHandler.SetVirtualView(IElement view)
at Microsoft.Maui.Controls.Platform.ShellView.CreateShellItemView()
at Microsoft.Maui.Controls.Platform.ShellView.SwitchShellItem(ShellItem newItem, Boolean animate)
at Microsoft.Maui.Controls.Handlers.ShellHandler.MapCurrentItem(ShellHandler handler, Shell view)
at Microsoft.Maui.PropertyMapper`2.<>c__DisplayClass5_0.<Add>b__0(IElementHandler h, IElement v)
at Microsoft.Maui.PropertyMapper.UpdatePropertyCore(String key, IElementHandler viewHandler, IElement virtualView)
at Microsoft.Maui.PropertyMapper.UpdateProperty(IElementHandler viewHandler, IElement virtualView, String property)
at Microsoft.Maui.Handlers.ElementHandler.UpdateValue(String property)
at Microsoft.Maui.Controls.Element.OnPropertyChanged(String propertyName)
at Microsoft.Maui.Controls.Shell.OnPropertyChanged(String propertyName)
at Microsoft.Maui.Controls.BindableObject.SetValueActual(BindableProperty property, BindablePropertyContext context, Object value, Boolean currentlyApplying, SetValueFlags attributes, Boolean silent)
at Microsoft.Maui.Controls.BindableObject.SetValueCore(BindableProperty property, Object value, SetValueFlags attributes, SetValuePrivateFlags privateAttributes)
at Microsoft.Maui.Controls.BindableObject.SetValueCore(BindableProperty property, Object value, SetValueFlags attributes)
at Microsoft.Maui.Controls.Element.SetValueFromRenderer(BindableProperty property, Object value)
at Microsoft.Maui.Controls.ShellNavigationManager.GoToAsync(ShellNavigationParameters shellNavigationParameters, ShellNavigationRequest navigationRequest)
at Microsoft.Maui.Controls.Shell.<Initialize>g__SetCurrentItem|172_1()
at Microsoft.Maui.Controls.Shell.<Initialize>b__172_0(Object s, NotifyCollectionChangedEventArgs e)
at System.Threading.Tasks.Task.<>c.<ThrowAsync>b__128_0(Object state)
at Microsoft.UI.Dispatching.DispatcherQueueSynchronizationContext.<>c__DisplayClass2_0.<Post>b__0()

I could reproduce this in a test by simply doing:

shell.CurrentItem = new ShellContent { Content = page };
shell.Items.Clear();
shell.CurrentItem = new ShellContent { Content = page };

I added appropriate null checking and the problem appears to be solved now.

[jonathanpeppers]

If this PR is merged before this one:

• Fixed removing items from the shell causing a crash #15098

I can update #15098 to fix the merge conflicts.

[PureWeen]

I think we need to store shell locally when the observer is set on line 75 that way we can successfully remove (this) from the list of AppearanceObservers. Otherwise, this Handler will probably remain inside the list of appearance observers inside shell.

maui/src/Controls/src/Core/Shell/Shell.cs

Line 313 in f6cf7c7

List<(IAppearanceObserver Observer, Element Pivot)> _appearanceObservers = new List<(IAppearanceObserver Observer, Element Pivot)>();

Or trigger this tear down code via

maui/src/Core/src/Platform/ActionDisposable.cs

Line 8 in f6cf7c7

class ActionDisposable : IDisposable

[jonathanpeppers]

It seems like this is a second problem.

I updated the test to check for this, and then fixed it. But if this fails on iOS/Android it's a problem in other handlers I didn't touch?