[listview] fixes for various null/empty DataTemplate (#13146) Fixes: #…

…11203

### Description of Change

Fixes: #11203

The following breaks XAML Hot Reload:

1. Setup a working `ListView` and `DataTemplate`

2. Delete the `DataTemplate`'s body (as if I'm about to type a
completely new view there).

3. Trigger a XAML Hot Reload. This can happen automatically if you just
pause typing.

4. MAUI crashes at runtime:

```
in Android.Views.ViewGroup.Layout at /Users/runner/work/1/s/xamarin-android/src/Mono.Android/obj/Release/net7.0/android-33/mcw/Android.Views.ViewGroup.cs:3369,5
in Microsoft.Maui.Controls.Handlers.Compatibility.VisualElementRenderer<Microsoft.Maui.Controls.ListView>.OnLayout at D:\a\_work\1\s\src\Controls\src\Core\Compatibility\Handlers\Android\VisualElementRenderer.cs:54,6
in Microsoft.Maui.Controls.Handlers.Compatibility.ListViewRenderer.OnLayout at D:\a\_work\1\s\src\Controls\src\Core\Compatibility\Handlers\ListView\Android\ListViewRenderer.cs:298,4
```

You can also create this problem in C#, like I did in a unit test:

listView.ItemTemplate = new DataTemplate(() => /* valid template */);
    listView.ItemTemplate = new DataTemplate(); // broken
    listView.ItemTemplate = new DataTemplate(() => null); //broken

I could also get a slightly different crash:

    System.InvalidCastException: Specified cast is not valid.
at
Microsoft.Maui.Controls.Internals.TemplatedItemsList`2[[Microsoft.Maui.Controls.ItemsView`1[[Microsoft.Maui.Controls.Cell,
Microsoft.Maui.Controls, Version=1.0.0.0, Culture=neutral,
PublicKeyToken=null]], Microsoft.Maui.Controls, Version=1.0.0.0,
Culture=neutral, PublicKeyToken=null],[Microsoft.Maui.Controls.Cell,
Microsoft.Maui.Controls, Version=1.0.0.0, Culture=neutral,
PublicKeyToken=null]].ActivateContent(Int32 index, Object item)

The "invalid cast" appears to be due to the changes in c57858f. It
returns a `new Label()` in some cases, which does not work with a
`ListView`. You need a `Cell` instead of `View` in that case.

The fixes appear to be in two places:

* `VisualElementRenderer.OnLayout`: use `is`, also simplifies the code

* `TemplatedItemsList.ActivateContent` : use `is` and call the default
data template.

Now my tests pass, and I can't reproduce the issue in XAML Hot Reload
either!

Manually testing, I'm able to alternative commenting out the two XAML
elements:


![image](https://user-images.githubusercontent.com/840039/217080323-113e4d5d-2b22-429c-87fa-c3bedbfb73de.png)


![image](https://user-images.githubusercontent.com/840039/217080982-1adea02f-a0eb-4ffd-8a67-922dc95d504f.png)

This didn't work before.

### Issues Fixed

Fixes: #11203

src/Compatibility/Core/src/Android/VisualElementRenderer.cs

@@ -300,10 +300,10 @@ protected virtual void OnElementPropertyChanged(object sender, PropertyChangedEv
 
 		protected override void OnLayout(bool changed, int l, int t, int r, int b)
 		{
-			if (Element == null)
-				return;
-
-			UpdateLayout(((IElementController)Element).LogicalChildren);
+			if (Element is IElementController controller)
+			{
+				UpdateLayout(controller.LogicalChildren);
+			}
 		}
 
 		public override void Draw(Canvas canvas)

src/Controls/src/Core/TemplatedItemsList.cs

@@ -534,8 +534,10 @@ public DataTemplate SelectDataTemplate(object item)
 
 		public TItem ActivateContent(int index, object item)
 		{
-			TItem content = ItemTemplate != null ? (TItem)ItemTemplate.CreateContent(item, _itemsView) : _itemsView.CreateDefault(item);
-
+			if (ItemTemplate?.CreateContent(item, _itemsView) is not TItem content)
+			{
+				content = _itemsView.CreateDefault(item);
+			}
 			content = UpdateContent(content, index, item);
 
 			return content;

src/Controls/tests/DeviceTests/Elements/ListView/ListViewTests.cs

@@ -20,6 +20,7 @@ void SetupBuilder()
 				builder.ConfigureMauiHandlers(handlers =>
 				{
 					handlers.AddHandler<ViewCell, ViewCellRenderer>();
+					handlers.AddHandler<TextCell, TextCellRenderer>();
 					handlers.AddHandler<ListView, ListViewRenderer>();
 					handlers.AddHandler<VerticalStackLayout, LayoutHandler>();
 					handlers.AddHandler<Label, LabelHandler>();
@@ -152,5 +153,61 @@ await CreateHandlerAndAddToWindow<LayoutHandler>(layout, async (handler) =>
 				await Task.Delay(100);
 			});
 		}
+
+		[Fact]
+		public async Task NullTemplateDoesntCrash()
+		{
+			SetupBuilder();
+			ObservableCollection<string> data = new ObservableCollection<string>()
+			{
+				"cat",
+				"dog",
+				"catdog"
+			};
+
+			var listView = new ListView(ListViewCachingStrategy.RecycleElement)
+			{
+				ItemTemplate = new DataTemplate(() =>
+				{
+					return new ViewCell
+					{
+						View = new VerticalStackLayout
+						{
+							new Label()
+						}
+					};
+				}),
+				ItemsSource = data
+			};
+
+			var layout = new VerticalStackLayout
+			{
+				listView
+			};
+
+			await CreateHandlerAndAddToWindow<LayoutHandler>(layout, async _ =>
+			{
+				await Task.Delay(100);
+				ValidatePlatformCells(listView);
+			});
+
+			// Default ctor
+			listView.ItemTemplate = new DataTemplate();
+
+			await CreateHandlerAndAddToWindow<LayoutHandler>(layout, async _ =>
+			{
+				await Task.Delay(100);
+				ValidatePlatformCells(listView);
+			});
+
+			// Return null
+			listView.ItemTemplate = new DataTemplate(() => null);
+
+			await CreateHandlerAndAddToWindow<LayoutHandler>(layout, async _ =>
+			{
+				await Task.Delay(100);
+				ValidatePlatformCells(listView);
+			});
+		}
 	}
 }